Add @Nullable to array element types that can hold null

Adds the JSpecify @Nullable annotation to the element type of a Java array whose elements are provably nullable, producing @Nullable String[] (the array's *elements* may be null) rather than String @Nullable [] (the array *reference* may be null), which NullAway checks in JSpecify mode. An array declaration (field, local, parameter, or return type) is annotated when its initializer is an array literal containing a null element (String[] a = {null}) or when an element is assigned a provably-null value (arr[i] = null) — within the same method body for a local or parameter, or anywhere in the enclosing class for a field (so a field array nulled in a separate method is still detected). Conservative by design: it skips primitive-element arrays, arrays whose element type already carries a nullability annotation, and non-Java sources.