fixes build error on Linux after its introduction in godotengine#18949

The function expects now a return value. Returning NULL seems to work in
this case.

(cherry picked from commit 6c74f3d)

modules/mono/editor/godotsharp_builds.cpp

@@ -129,12 +129,12 @@ MonoString *godot_icall_BuildInstance_get_MSBuildPath() {
 	if (build_tool == GodotSharpBuilds::XBUILD) {
 		if (xbuild_path.empty()) {
 			WARN_PRINT("Cannot find binary for '" PROP_NAME_XBUILD "'");
-			return;
+			return NULL;
 		}
 	} else {
 		if (msbuild_path.empty()) {
 			WARN_PRINT("Cannot find binary for '" PROP_NAME_MSBUILD_MONO "'");
-			return;
+			return NULL;
 		}
 	}