Hide empty spaces where validation is enabled but nothing to show

[andydotxyz]

This includes a slight change in behaviour that errors would be shown after initial input is completed.
It is helpful to avoid forms jumping around when focus changes.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 62.69% (+0.01%) from 62.676%
when pulling 0aa8f06 on andydotxyz:fix/formspacing
into 4e77e09 on fyne-io:develop.

[dweymouth]

Does this cause "jumps" in the layout positioning when a validation error is shown then? If so, I wonder if it should be somehow themeable because some may prefer the compact layout while others prefer no layout jumps?

[andydotxyz]

Yes if and only if an error appears with no hint. If a developer wants to prop it open they could use " " as a hint.

[dweymouth]

Yes if and only if an error appears with no hint. If a developer wants to prop it open they could use " " as a hint.

Good point that " " can prop it open if desired

[andydotxyz]

Does that mean we're good @dweymouth ?