This fixes Mantis bug 437.
compiler/split_parse_tree_src.m:
Add empty submodules to the submodules map, since the abort
was caused absence of such an entry.
Warn about the duplication if a submodule is later defined
with the same name.
tests/valid/empty_submodule.m:
Add this regression test for the bug.
tests/valid/Mmakefile:
Enable the new test case.
