[5.x] Make the SVG tag fail gracefully when src value is empty

[duncanmcclean]

This pull request makes it so the {{ svg }} tag fails gracefully when the provided src value is empty. This matches how the SVG tag handled empty src values in v4.

The example given in #9902, was that of an icon field without any value:

{{ svg src="{{ foo }}" }}

Fixes #9902.