qt5 module generates broken target if moc-qt5 is not installed

[thiblahute]

When building gst-plugins-base while moc-qt5 is not installed on the system I get:

Traceback (most recent call last):
  File "/home/jenkins/workspace/meson/mesonbuild/mesonmain.py", line 282, in run
    app.generate()
  File "/home/jenkins/workspace/meson/mesonbuild/mesonmain.py", line 169, in generate
    g.generate(intr)
  File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 184, in generate
    self.generate_target(t, outfile)
  File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 257, in generate_target
    self.generate_custom_generator_rules(target, outfile)
  File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 1360, in generate_custom_generator_rules
    self.generate_genlist_for_target(genlist, target, outfile)
  File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 1406, in generate_genlist_for_target
    elem.write(outfile)
  File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 116, in write
    if quote_char == '"':
  File "/home/jenkins/workspace/meson/mesonbuild/backend/ninjabackend.py", line 35, in ninja_quote
    return text.replace(' ', '$ ').replace(':', '$:')
AttributeError: 'NoneType' object has no attribute 'replace'

basically because the target genrated here is broken with None as first argument