I'd like to build a custom number format where the text format section
contains an if statement. Here is an example to explain what I'd like
to replicate:

#,##0_);(#,##0);;if="null" then ""

The number format would leave treat positive and negative values
normally, would display a blank cell if the value is zero, and would
display a blank cell if the cell value is text "null", otherwise if
the cell value is any other text it would display that text.

1,-2,0,null,X would become 1,-2,,,X

I'm not sure if what I am asking makes sence, so please let me know if
I can clarify my question.

What I want is an example of a custom number format that would do
this. If even possible.

Thanks in advance,
TK