Trim Empty PLMNs from the list of parsed PLMNs

aosp/428381/ changed the PLMN parsing in IccUtils
to trim all 'f' characters, which resulted in a few
functions returning empty instead of ffffff for SIM
records that were unfilled. To fix those methods,
we now trim all results that are returned as empty

Bug: 68218015
Test: cts - testGetForbiddenPlmns()
Change-Id: I07fa0f2ef3d0135f672bd50b5079d23e46cce636