Ignore a linedef tag of 0 when using linedef action 96.

When using action 96, Apply Tag to Tagged Sectors, leaving the linedef tag as 0 means that every sector with tag 0 will gain the target tags. Considering tag 0 is usually treated as having no tag, and the tags set via the offset fields already ignore 0, this seemed like undesirable and inconsistent behavior to me.

However, this could potentially remove some niche use cases that I haven't thought of yet. Please leave a comment if this is the case!