Force backup analog traces only if needed

Force backup analog traces only if needed

With the current conditions, when using the 'srzip' output module with
device generating SR_DF_ANALOG packets, they are processed both by
'srzip' and the backup 'analog' module (as 'out' is always null for
srzip which does its own file operation).
For a moderately fast continuous analog capture (e.g. 2.4MSamples/s),
the overhead of the 'analog' module is important (e.g. 80% of the whole
processing of my workstation) while we are not using its output,
so disable it when the user actually selected an output format/file
but the module doesn't generate text output.

Also remove the second part of the condition as 'opt_output_format'
never seems to be null or at least not in the case where the user has
omitted output parameters.

Tested with a device generating analog packets both with '-o test.sr'
and no output option.