It is sometimes fairly complicated to skip a template value on a parameter when using a {{#tag:mapframe | | <if ... then "show=..." else "nothing"> }}, so we should allow show= to be an empty value, to mean the same thing as no value. Same goes for the group=, and possibly other optional parameters. Note that text= without a value could have a specific meaning of "don't generate any HTML, but still add the geojson to the page's data".