This isn't a high priority issue but, came up when coding a template here: https://en.wikisource.org/w/index.php?title=Template:Ukroadsign/images&action=history

It wasn't possible in the clause for image=@dia to do a simple {{{newno|{{{diagram}}} }}} paramater as certain documentation suggested, because newno is conditionally passed in from a higher level template https://en.wikisource.org/w/index.php?title=Template:Ukroadsign as

{{{newno|}}}

meaning that an empty string "" is generated, rather than what was intended which was that if newno wasn't supplied to the parent it wasn't passed onto the sub template.

It would be nice to have a way of more precisely doing the intended logic which is if {{{newno}}} is unknown to the parent, don't pass it to the sub-template, WITHOUT having to make the {{{newno}}} paramater mandatory in the parent template.

Thusly it would be appreciated if a #null: value could be explicitly specified. In the parent template I can then write

newno={{{newno|#null}}}

which the relevant clause in the called template

{{{newno|{{{diagram}}} }}}

will interpret correctly, applying {{{newno}}} if it's specified, or {{{diagram}}} if it's null or unspecified.

Explicitly specifying nulls in this manner would in some places make it easier to write certain templates or at least debug them.