Vis fixes3

[stephanmg]

Empty fields (e.g. for plotName) lead to 'nan' value for title of plot.

Therefore changed empty fields to a field entry with the space character.

[stephanmg]

Potentially only relevant for rather old petab versions, tested version with this issue is <=0.5.0.

[plakrisenko]

No files are changed

[dilpath]

Should this be fixed in the plotting code instead?

[stephanmg]

Should this be fixed in the plotting code instead?

I think the issue is only reproducible in older petab versions - so, we could also discard my changes and argue not to use older petab versions).