simd module: fix regression that broke using only some simd variants

Regression in commit a3d287c553.

When a given kwarg is not specified, we want to not generate it as a
simd variant. Since the default for buildtarget sources is `[]` it
resulted in building a static library with no sources, and a warning
stating that this was buggy and will eventually be removed.

Fix this by teaching buildtarget sources to allow None, and defaulting
to it specifically for the simd module. We can check this and then skip
processing entirely.

Fixes #12438

mesonbuild/interpreter/type_checking.py

@@ -470,7 +470,7 @@ SOURCES_VARARGS = (str, File, CustomTarget, CustomTargetIndex, GeneratedList, St
 
 BT_SOURCES_KW: KwargInfo[SourcesVarargsType] = KwargInfo(
     'sources',
-    ContainerTypeInfo(list, SOURCES_VARARGS),
+    (NoneType, ContainerTypeInfo(list, SOURCES_VARARGS)),
     listify=True,
     default=[],
 )

mesonbuild/modules/simd.py

@@ -71,7 +71,7 @@ class SimdModule(ExtensionModule):
     @typed_pos_args('simd.check', str)
     @typed_kwargs('simd.check',
                   KwargInfo('compiler', Compiler, required=True),
-                  *[BT_SOURCES_KW.evolve(name=iset) for iset in ISETS],
+                  *[BT_SOURCES_KW.evolve(name=iset, default=None) for iset in ISETS],
                   *[a for a in STATIC_LIB_KWS if a.name != 'sources'],
                   allow_unknown=True) # Because we also accept STATIC_LIB_KWS, but build targets have not been completely ported to typed_pos_args/typed_kwargs.
     @permittedKwargs({'compiler', *ISETS, *build.known_stlib_kwargs}) # Also remove this, per above comment
@@ -90,6 +90,8 @@ class SimdModule(ExtensionModule):
 
         for iset in ISETS:
             sources = kwargs[iset]
+            if sources is None:
+                continue
 
             compile_args = compiler.get_instruction_set_args(iset)
             if compile_args is None: