modules/pkgconfig: Don't insert None into devenv list

When the pkgconfig module is imported, but not used, it will insert None
on the end of the devenv list. This list is not expected to contain
None, and causes Meson to crash. This can happen in complex build setups
(reported from mesa), where pkgconfig is only used in some
configurations

Fixes: #12032

mesonbuild/modules/pkgconfig.py

@@ -391,7 +391,8 @@ class PkgConfigModule(NewExtensionModule):
         })
 
     def postconf_hook(self, b: build.Build) -> None:
-        b.devenv.append(self.devenv)
+        if self.devenv is not None:
+            b.devenv.append(self.devenv)
 
     def _get_lname(self, l: T.Union[build.SharedLibrary, build.StaticLibrary, build.CustomTarget, build.CustomTargetIndex],
                    msg: str, pcfile: str) -> str:

test cases/unit/90 devenv/meson.build

@@ -20,3 +20,6 @@ env = environment({'TEST_C': ['/prefix']}, method: 'prepend')
 meson.add_devenv(env)
 env = environment({'TEST_C': ['/suffix']}, method: 'append')
 meson.add_devenv(env)
+
+# Reproducer for https://github.com/mesonbuild/meson/issues/12032
+pkgconf = import('pkgconfig')