mw.loader: Return consistent object shape from splitModuleKey()

mw.loader: Return consistent object shape from splitModuleKey()

This is used in mw.loader.implement() in a truthy check before
assigning as value of `registry[module].version`.

Both implicit undefined and empty string are falsey so the
behaviour is unchanged.

This reduces shape variability for JS engines, and also avoid
potential mistakes if the function is used elsewhere since
internally the registry, as maintained by mw.loader.register(),
uses empty string as default for version, not undefined.

Change-Id: I067137557a55d521fefa40986373e55b6bbb59ff