util: Make sure to handle special empty strings

util: Make sure to handle special empty strings

Some SIMs contain an EFspn with the contents all set to 'filler'
characters, e.g. 0xFF.  We mistakenly do not handle these strings
correctly.

Aug  8 11:40:00 mx31tt01 daemon.info ofonod[622]: Aux: >
AT+CRSM=176,28486,0,0,17\r
Aug  8 11:40:00 mx31tt01 daemon.info ofonod[622]: Aux: < \r\n+CRSM:
144,0,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF\r\n\r\nOK\r\n
Aug  8 11:40:00 mx31tt01 daemon.debug ofonod[622]:
drivers/atmodem/sim.c:at_crsm_read_cb() crsm_read_cb: 90, 00, 17
Aug  8 11:40:00 mx31tt01 daemon.debug ofonod[622]:
src/simfs.c:sim_fs_op_read_block_cb() bufoff: 0, dataoff: 0, tocopy: 17
Aug  8 11:40:00 mx31tt01 daemon.err ofonod[622]: EFspn read
successfully, but couldn't parse