should throw an error if biophysical_neuron_models_dir is missing for biophysical nodes

[mgeplf]

it is needed only for biophysical populations: "Path to the template HOC files defining the E-Model. There must be one defined for biophysical node populations.

[joni-herttuainen]

It seems that the error is not thrown in a case in which the populations dict for a given population is empty.
I.e., Assume components is defined (without biophysical_neuron_models_dir) as

"components": {
    "morphologies_dir": "$MORPH_DIR",
}

then an error is not thrown for this node population named default (that will default to type = 'biophysical')

"populations" : {
    "default": {}
}

but is for this:

"populations" : {
    "default": {
        "test": "value"
    }
}

This came up in SNAP tests. In the spec it's defined that

The dictionary associated to each population may be empty if the components properties don’t need to be overridden