actually return the empty string for com.nokia.mid.imei

*System.getProperty0* is supposed to return an empty string for *com.nokia.mid.imei*, but it actually returns the `null` value, because it checks the return value for truthiness, and an empty string is falsy.

This fix makes it return the empty string (and tests that it does so).  It requires that each *case* block explicitly set *value* to the desired value, even if that's `null` (most were already doing so, but *microedition.platformimpl* needed to be updated to set the value explicitly). It also makes the same change to *Configuration.getProperty0*.