[NETBEANS-7949] Fixing handling of 'case null'.

[lahodaj]

This tries to handle various cases related to case null:

• if the selector is nullable, it still won't throw an exception
• if the selector is a variable, the variable will definitely be null inside case null and definitely not-null in other cases
• proper handling of { yield ...; }

[mbien]

makes sense to me

[mbien]

nitpick: one of my jackpot rules matched here, shorter version which combines the last three stages:

        boolean hasNullCase = cases.stream()
                                   .flatMap(ct -> ct.getLabels().stream())
                                   .filter(clt -> clt.getKind() == Kind.CONSTANT_CASE_LABEL)
                                   .map(ctl -> ((ConstantCaseLabelTree) ctl).getConstantExpression())
                                   .anyMatch(expr -> expr.getKind() == Kind.NULL_LITERAL);