Refine null-safety in the spring-test module

Closes gh-34161

spring-web/src/main/java/org/springframework/http/InvalidMediaTypeException.java

@@ -51,6 +51,14 @@ public class InvalidMediaTypeException extends IllegalArgumentException {
 		this.mediaType = ex.getMimeType();
 	}
 
+	/**
+	 * Returns the detail message string of this exception instance (never {@code null}).
+	 */
+	@Override
+	@SuppressWarnings("NullAway") // InvalidMediaTypeException message is never null
+	public String getMessage() {
+		return super.getMessage();
+	}
 
 	/**
 	 * Return the offending media type.