Support case-insensitive null literals in SpEL

Prior to this commit null literals in SpEL expressions had to be
specified as "null" (i.e., all lowercase).

With this commit null literals in SpEL expressions are interpreted in a
case-insensitive manner, analogous to the current support for boolean
literals.

Issue: SPR-9613

spring-expression/src/main/java/org/springframework/expression/spel/standard/InternalSpelExpressionParser.java

@@ -513,7 +513,7 @@ class InternalSpelExpressionParser extends TemplateAwareExpressionParser {
 	private boolean maybeEatNullReference() {
 		if (peekToken(TokenKind.IDENTIFIER)) {
 			Token nullToken = peekToken();
-			if (!nullToken.stringValue().equals("null")) {
+			if (!nullToken.stringValue().toLowerCase().equals("null")) {
 				return false;
 			}
 			nextToken();