Cannot remove the box macro title once set

Follow this steps:

• open the Insert Macro modal
• select the Box macro
• insert without setting the title parameter
• at this point the box title is not visible in-line so it can't be edited, which is expected
• open the Edit Macro modal and fill the title parameter
• after this the box title is visible and editable in-line, which is expected
• now suppose you want to remove the box title, the first thing you try is to empty the box title in-line
  • if you switch to source you'll see that the title parameter is still present, but with a white space
  • if you save it looks OK (title is not displayed) but if you edit back you can still edit the box title in-line (because of the white space in its value)
  • if you open the Edit Macro modal you don't get the title field because it is editable in-line (which is expected) so there's no way to completely remove the title parameter except for switching to source.

This is minor because in view mode it looks OK, but maybe with other macros there will be a difference (i.e. the whitespace in the parameter value might be significant). Basically we need a way to empty / remove an optional macro parameter that is being editable in-line.