res_musiconhold: Avoid spurious warning when 'format' is the empty string

The change to res_config_odbc that allowed empty strings to be
returned to realtime consumers¹ causes a warning to be emitted when
loading MoH classes. So we need to treat an empty 'format' as if it
was not specified to avoid the warning.

ASTERISK-28735 #close
Reported by: Ross Beer

[1] https://gerrit.asterisk.org/c/asterisk/+/13722

Change-Id: I9a271d721e1a0973e80ebe7d75b46a0d8fa0e5a5