Reintroduce support for null SpEL expressions

[github-actions]

Overview

In gh-30371, @ryan-c-wicks brought it to our attention that null SpEL expressions were unintentionally supported prior to #30330.

For example, the following test passed.

	@Test
	void nullExpression() {
		ExpressionParser parser = new SpelExpressionParser();
		String expression = null;
		Expression expr = parser.parseExpression(expression);
		Object result = expr.getValue();
		assertThat(result).isNull();
	}

Since the 5.2.x branch is no longer actively maintained, we have decided to treat this as a "regression" for 5.2.x and will update the code to avoid a NullPointerException whenever the user supplies null as the SpEL expression.

Related Issues

• Limit SpEL expression length #30330
• Reject null and empty SpEL expressions #30371

[sbrannen]

Closed via 89a3d64