The trick is to have a parameter that defaults to blank (or any other value that nobody can use as a valid number to be multiplied, often called a sentinel value in programming), and create a first filter run that removes the parameter from the pipeline (making it empty) if the parameter is still set to the sentinel value. If you still have a value, then the user of the function supplied a parameter and you can multiply it by two.

That settles the parameter case. Now for the input tiddler case: use the :else (also spelled ~) filter run prefix to supply a second run which is used only if the first run had no results (thus no parameter was passed). In this run, multiply the input tiddler by 2.

All together:

\function multiply-anything-by-two(number:"") [<number>!is[blank]multiply[2]] :else[multiply[2]]

* {{{ [[4]function[multiply-anything-by-two]] }}} should equal 8.
* {{{ [function[multiply-anything-by-two],[4]] }}} should equal 8.