[fix] Parse array is None if CFG conditionals when quering for memlets

[FlorianDeconinck]

We encounter a case of CFG within a NestedSDFG where the condition is of the form

array is None

and where array is a legit optional parameter. The SDFG -> STREE (and other) system relies on parsing the conditional AST to extract the arrays and propagate the memlets properly. The code ends up in dace.sdfg.memlets_in_ast where it is stated that the limitations are:

• no indirect arrays (expecting to be able to use symbolic expression)
• scalars

We propose here to add a narrow parse of array is None to strengthen the list. Implementation restrict the parsing to a single form to evade run away checks.

[tbennun]

Very minor change, otherwise lgtm

[acalotoiu]

LGTM