res_format_attr_opus: Fix fmtp generation.

res_format_attr_opus assumed that the string being passed into it was
empty. It tried to determine if the only thing it had written was

a=fmtp:<num>

And if it had, it would reset the string. Its calculation was off when
working with chan_sip, though. chan_sip passes the entire built SDP
rather than an empty string. This resulted in always putting an empty
fmtp line in the SDP.

ASTERISK-26520 #close
Reported by scgm11

Change-Id: Ib2e8712d26a47067e5f36d5973577added01dbb5