PipeWireSourceStream: work around crashes for pipewire < 0.3.49
Before 0.3.49 pw_stream_update_params will crash when the param list has nullptr, but Neon only has 0.3.48. This adds a check to make sure the param list doesn't have nullptr to fix the crash.
BUG: 492400 SENTRY: PLASMA-WORKSPACE-1R8 FIXED-IN: 6.2.1