use_null_aware_elements

Use the null-aware marker '?' rather than a null check via an 'if'.

The analyzer produces this diagnostic when a null check is used instead of a null-aware marker inside of a collection literal.

The following code produces this diagnostic because a null check is used to decide whether x should be inserted into the list, while the null-aware marker '?' would be less brittle and less verbose.

f(int? x) => [if (x != null) x];

Replace the null-check with the null-aware marker '?':

f(int? x) => [?x];