mw.Map: Check presence of value argument in set()

mw.Map: Check presence of value argument in set()

Add back the `arguments.length > 1` check (accidentally removed
in 24f84b0). Otherwise it inadvertently uses the `value`
parameter, causing it to set undefined as the value.

There was already a test ensuring undefined can be set as value,
but a test to ensure it doesn't default to undefined was missing.

Change-Id: I4c69f0c11f165640a9b387a72c77c48eb6aa9e72