[BUG] Math Block `$$ $$` inside proof directive throws error

[AakashGfude]

Using $$ $$ for math block throws error for absence of id.
The error is specifically coming when sphinx tries to number the equation

[AakashGfude]

My bad, the error comes for math_blocks with targets like in this example:

[najuzilu]

Thanks for the feedback @AakashGfude. This is an issue with the proof:proof directive.

[mmcky]

• add as a test case when writing a fix