Reject null and empty SpEL expressions

Prior to gh-30325, supplying a null reference for a SpEL expression was
effectively equivalent to supplying the String "null" as the
expression. Consequently, evaluation of a null reference expression
always evaluated to a null reference. However, that was accidental
rather than by design.

Due to the introduction of the checkExpressionLength(String) method in
InternalSpelExpressionParser (in conjunction with gh-30325), an attempt
to evaluate a null reference as a SpEL expression now results in a
NullPointerException.

To address both of these issues,
TemplateAwareExpressionParser.parseExpression() and
SpelExpressionParser.parseRaw() now reject null and empty SpEL
expressions.

Closes gh-30371

spring-expression/src/test/java/org/springframework/expression/spel/TemplateExpressionParsingTests.java

@@ -30,6 +30,7 @@ import org.springframework.expression.spel.support.StandardEvaluationContext;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * @author Andy Clement
@@ -43,6 +44,12 @@ class TemplateExpressionParsingTests extends AbstractExpressionTests {
 	private final SpelExpressionParser parser = new SpelExpressionParser();
 
 
+	@Test
+	void nullTemplateExpressionIsRejected() {
+		assertThatIllegalArgumentException()
+			.isThrownBy(() -> parser.parseExpression(null, DOLLAR_SIGN_TEMPLATE_PARSER_CONTEXT))
+			.withMessage("'expressionString' must not be null");
+	}
 
 	@Test
 	void parsingSimpleTemplateExpression01() {

spring-expression/src/test/java/org/springframework/expression/spel/standard/SpelParserTests.java

@@ -33,6 +33,7 @@ import org.springframework.expression.spel.support.StandardEvaluationContext;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.springframework.expression.spel.SpelMessage.MISSING_CONSTRUCTOR_ARGS;
 import static org.springframework.expression.spel.SpelMessage.NON_TERMINATING_DOUBLE_QUOTED_STRING;
 import static org.springframework.expression.spel.SpelMessage.NON_TERMINATING_QUOTED_STRING;
@@ -53,6 +54,32 @@ class SpelParserTests {
 	private final SpelExpressionParser parser = new SpelExpressionParser();
 
 
+	@Test
+	void nullExpressionIsRejected() {
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseExpression(null));
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseRaw(null));
+	}
+
+	@Test
+	void emptyExpressionIsRejected() {
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseExpression(""));
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseRaw(""));
+	}
+
+	@Test
+	void blankExpressionIsRejected() {
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseExpression("     "));
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseExpression("\t\n"));
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseRaw("     "));
+		assertNullOrEmptyExpressionIsRejected(() -> parser.parseRaw("\t\n"));
+	}
+
+	private static void assertNullOrEmptyExpressionIsRejected(ThrowingCallable throwingCallable) {
+		assertThatIllegalArgumentException()
+			.isThrownBy(throwingCallable)
+			.withMessage("'expressionString' must not be null or blank");
+	}
+
 	@Test
 	void theMostBasic() {
 		SpelExpression expr = parser.parseRaw("2");