Don't crash when asked to enable an AP with a null SSID.

WifiConfiguration can have been build by an app with a NULL SSID, or
other null fields. We need to verify that at least that those fields
are not NULL before using them. More specifically, don't crash when
enabling an AP with a null SSID in setApConfiguration function.

Bug: 14568953
Change-Id: I6b33ddf97789c0ee4b850d3ec0bccb08100c21d5