Merge pull request #9003 from Microsoft/caseUndefinedAndNull

Allow case comparison to undefined and null in strict null checking mode

src/compiler/checker.ts

@@ -15110,7 +15110,7 @@ namespace ts {
                     // In a 'switch' statement, each 'case' expression must be of a type that is comparable
                     // to or from the type of the 'switch' expression.
                     const caseType = checkExpression(caseClause.expression);
-                    if (!isTypeComparableTo(expressionType, caseType)) {
+                    if (!isTypeEqualityComparableTo(expressionType, caseType)) {
                         // expressionType is not comparable to caseType, try the reversed check and report errors if it fails
                         checkTypeComparableTo(caseType, expressionType, caseClause.expression, /*headMessage*/ undefined);
                     }