Fix return type of debugfile functions

It was returning None in some cases while being marked as returning
string.

mesonbuild/backend/ninjabackend.py

@@ -2797,7 +2797,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         else:
             return compiler.get_compile_debugfile_args(objfile, pch=False)
 
-    def get_link_debugfile_name(self, linker, target, outname):
+    def get_link_debugfile_name(self, linker, target, outname) -> T.Optional[str]:
         return linker.get_link_debugfile_name(outname)
 
     def get_link_debugfile_args(self, linker, target, outname):

mesonbuild/compilers/compilers.py

@@ -921,7 +921,7 @@ class Compiler(HoldableObject, metaclass=abc.ABCMeta):
     def get_compile_debugfile_args(self, rel_obj: str, pch: bool = False) -> T.List[str]:
         return []
 
-    def get_link_debugfile_name(self, targetfile: str) -> str:
+    def get_link_debugfile_name(self, targetfile: str) -> T.Optional[str]:
         return self.linker.get_debugfile_name(targetfile)
 
     def get_link_debugfile_args(self, targetfile: str) -> T.List[str]:

mesonbuild/compilers/mixins/islinker.py

@@ -120,8 +120,8 @@ class BasicLinkerIsCompilerMixin(Compiler):
     def get_buildtype_linker_args(self, buildtype: str) -> T.List[str]:
         return []
 
-    def get_link_debugfile_name(self, targetfile: str) -> str:
-        return ''
+    def get_link_debugfile_name(self, targetfile: str) -> T.Optional[str]:
+        return None
 
     def thread_flags(self, env: 'Environment') -> T.List[str]:
         return []

mesonbuild/linkers/linkers.py

@@ -86,7 +86,7 @@ class StaticLinker:
     def native_args_to_unix(cls, args: T.List[str]) -> T.List[str]:
         return args[:]
 
-    def get_link_debugfile_name(self, targetfile: str) -> str:
+    def get_link_debugfile_name(self, targetfile: str) -> T.Optional[str]:
         return None
 
     def get_link_debugfile_args(self, targetfile: str) -> T.List[str]:
@@ -428,7 +428,7 @@ class DynamicLinker(metaclass=abc.ABCMeta):
     def has_multi_arguments(self, args: T.List[str], env: 'Environment') -> T.Tuple[bool, bool]:
         raise EnvironmentException(f'Language {self.id} does not support has_multi_link_arguments.')
 
-    def get_debugfile_name(self, targetfile: str) -> str:
+    def get_debugfile_name(self, targetfile: str) -> T.Optional[str]:
         '''Name of debug file written out (see below)'''
         return None