build: fix some type annotations for get_install_dir

mesonbuild/build.py

@@ -570,10 +570,11 @@ class Target(HoldableObject, metaclass=abc.ABCMeta):
     def get_custom_install_dir(self) -> T.List[T.Union[str, Literal[False]]]:
         raise NotImplementedError
 
-    def get_install_dir(self) -> T.Tuple[T.List[T.Union[str, Literal[False]]], str, Literal[False]]:
+    def get_install_dir(self) -> T.Tuple[T.List[T.Union[str, Literal[False]]], T.List[T.Optional[str]], bool]:
         # Find the installation directory.
         default_install_dir, default_install_dir_name = self.get_default_install_dir()
-        outdirs = self.get_custom_install_dir()
+        outdirs: T.List[T.Union[str, Literal[False]]] = self.get_custom_install_dir()
+        install_dir_names: T.List[T.Optional[str]]
         if outdirs and outdirs[0] != default_install_dir and outdirs[0] is not True:
             # Either the value is set to a non-default value, or is set to
             # False (which means we want this specific output out of many