0.45.0: unset variable breaks build (due to imagemagick not being available)

Created by: dvzrv

Describe the bug

Setting top to                           : /build/element/src/element
Setting out to                           : /build/element/src/element/build
Checking for program 'ccache'            : not found
Checking for program 'clang'             : not found
Checking for 'gcc' (C compiler)          : /usr/bin/gcc
Checking for 'g++' (C++ compiler)        : /usr/bin/g++
Checking for program 'ar'                : /usr/bin/ar
Checking for compiler flags ['-std=c++17'] : yes
Checking for program 'convert'           : not found
Checking for library curl                : yes
Checking for header stdbool.h            : yes
Checking for header boost/signals2.hpp   : yes
Checking for program 'pkg-config'        : /usr/bin/pkg-config
Checking for 'jack'                      : yes
Checking for header pluginterfaces/vst2.x/aeffect.h : not found
Checking for 'lv2'                       : yes
Checking for 'lilv-0'                    : yes
Checking for 'suil-0'                    : yes
Checking for suil_init(...)              : yes
Checking for library pthread             : yes
Checking for library dl                  : yes
Checking for library readline            : yes
Checking for header curl/curl.h          : yes
Checking for library curl                : yes
Checking for header ladspa.h             : yes
Checking for 'alsa'                      : yes
Checking for 'freetype2'                 : yes
Checking for 'x11'                       : yes
Checking for 'xext'                      : yes
Checking for 'xrandr'                    : yes
Checking for 'xcomposite'                : yes
Checking for 'xinerama'                  : yes
Checking for 'xcursor'                   : yes
Checking for 'gtk+-2.0'                  : yes
Element
 * ALSA                                  : True
 * JACK                                  : True
 * AU                                    : False
 * VST2                                  : False
 * VST3                                  : True
 * LADSPA                                : True
 * LV2                                   : True
 * GtkUI                                 : True
 * Lua                                   : True
 * Workspaces                            : True
 * Debug                                 : False
 * PREFIX                                : /usr
 * DATADIR                               : /usr/share/element
 * CFLAGS                                : ['-march=x86-64', '-mtune=generic', '-O2', '-pipe', '-fno-plt', '-Os', '-Wno-deprecated-declarations']
 * CXXFLAGS                              : ['-march=x86-64', '-mtune=generic', '-O2', '-pipe', '-fno-plt', '-Os', '-std=c++17', '-Wno-deprecated-declarations']
 * LINKFLAGS                             : []
'configure' finished successfully (1.950s)
Waf: Entering directory `/build/element/src/element/build'
Traceback (most recent call last):
  File "/lib/python3.8/site-packages/waf/waflib/Scripting.py", line 159, in waf_entry_point
    run_commands()
  File "/lib/python3.8/site-packages/waf/waflib/Scripting.py", line 255, in run_commands
    ctx = run_command(cmd_name)
  File "/lib/python3.8/site-packages/waf/waflib/Scripting.py", line 239, in run_command
    ctx.execute()
  File "/lib/python3.8/site-packages/waf/waflib/Scripting.py", line 582, in execute
    return execute_method(self)
  File "/lib/python3.8/site-packages/waf/waflib/Build.py", line 231, in execute
    self.execute_build()
  File "/lib/python3.8/site-packages/waf/waflib/Build.py", line 244, in execute_build
    self.recurse([self.run_dir])
  File "/lib/python3.8/site-packages/waf/waflib/Context.py", line 296, in recurse
    user_function(self)
  File "/build/element/src/element/wscript", line 234, in build
    compile (bld)
  File "/build/element/src/element/wscript", line 219, in compile
    build_desktop (bld)
  File "/build/element/src/element/wscript", line 133, in build_desktop
    if len (bld.env.CONVERT[0]) > 0:
IndexError: list index out of range

To Reproduce Steps to reproduce the behavior:

1. waf configure --prefix=/usr --enable-docking --test
2. waf build -vv
3. See message above

Expected behavior Build happens without using convert or build throws a more clear error message about why it is failing

Screenshots n/a

Desktop (please complete the following information):

• OS: Arch Linux
• Version n/a

Standalone or Plugin build

Additional context n/a