Fix java code generator emitting unkown None symbol

Quite similar to 6d85020
and fixing building the java code for gxformat2.

Author: mvdbeek

schema_salad/java_codegen.py

@@ -471,7 +471,9 @@ def type_loader(
                         name=f"map_of_{i.name}",
                         init="new MapLoader({}, {}, {})".format(
                             i.name,
-                            f"'{container}'" if container is not None else None,  # noqa: B907
+                            (
+                                f"'{container}'" if container is not None else self.to_java(None)
+                            ),  # noqa: B907
                             self.to_java(no_link_check),
                         ),
                         loader_type=f"Loader<java.util.Map<String, {i.instance_type}>>",
@@ -494,7 +496,9 @@ def type_loader(
                             clazz=fqclass,
                             ext="Impl" if not is_abstract else "",
                             container=(
-                                f"'{container}'" if container is not None else None  # noqa: B907
+                                f"'{container}'"
+                                if container is not None
+                                else self.to_java(None)  # noqa: B907
                             ),
                             no_link_check=self.to_java(no_link_check),
                         ),