Fix crash with empty transclusions

[Jermolene]

This PR addresses infinite loops that can occur when {{}} is typed in the editor with the preview displayed. See #8144 for an example, but this is a long standing problem. See #7768 for an earlier attempt to fix this problem

The approach taken here is to modify the parse rules for both inline and block transclusions so that the transclusion is not recognised unless there is a non-blank target and/or template.