This is especially true since procedures are defined by plug-ins,
possibly third-party. So they may not have a first run mode parameter.
Also we don't want plug-ins to generate CRITICALs in the core
application.
Therefore before trying to get and/or set the run mode parameter, always
check that this first parameter exists, and if so, is it a GimpRunMode
enum?

Fixes similar errors:
> GIMP-CRITICAL: gimp_value_array_index: assertion 'index < value_array->n_values' failed

Moreover it also makes sense to allow plug-ins to not have such
parameter. What if you make a plug-in which always work in a single mode
and don't ever plan/want to make it work interactively for instance?

Lastly old Python 2 plug-ins didn't even have a run mode parameter (I
guess our old Python 2 wrapper was getting rid of it?).