Fix default file parsing of three options

The default file should only be used if an option is not specified on
the command line, but instead, it was only used if the option *was*
specified (overriding the specified value).

mkosi

@@ -1766,10 +1766,10 @@ def process_setting(args, section, key, value):
             else:
                 args.extra_trees.extend(list_value)
         elif key == "BuildScript":
-            if args.build_script is not None:
+            if args.build_script is None:
                 args.build_script = value
         elif key == "BuildSources":
-            if args.build_sources is not None:
+            if args.build_sources is None:
                 args.build_sources = value
         elif key == "BuildPackages":
             list_value = value if type(value) == list else value.split()
@@ -1781,7 +1781,7 @@ def process_setting(args, section, key, value):
             if args.postinst_script is None:
                 arg.postinst_script = value
         elif key == "NSpawnSettings":
-            if args.nspawn_settings is not None:
+            if args.nspawn_settings is None:
                 args.nspawn_settings = value
         elif key is None:
             return True