backend: Headers.install_subdir is allowed to be None

But we don't properly handle that.

mesonbuild/backend/backends.py

@@ -1445,7 +1445,12 @@ class Backend:
         for h in headers:
             outdir = h.get_custom_install_dir()
             if outdir is None:
-                outdir = os.path.join(incroot, h.get_install_subdir())
+                subdir = h.get_install_subdir()
+                if subdir is None:
+                    outdir = incroot
+                else:
+                    outdir = os.path.join(incroot, subdir)
+
             for f in h.get_sources():
                 if not isinstance(f, File):
                     raise MesonException(f'Invalid header type {f!r} can\'t be installed')

mesonbuild/build.py

@@ -149,7 +149,7 @@ class Headers(HoldableObject):
     def set_install_subdir(self, subdir: str) -> None:
         self.install_subdir = subdir
 
-    def get_install_subdir(self) -> str:
+    def get_install_subdir(self) -> T.Optional[str]:
         return self.install_subdir
 
     def get_sources(self) -> T.List[File]: