Restore StringUtils.hasLength check

Update `MimeTypeUtils` so that the  StringUtils.hasLength check is
performed immediately on the incoming argument, rather than in
`parseMimeTypeInternal`. This restores the `IllegalArgumentException`
rather than the `NullPointerException` which is thrown by the
`ConcurrentHashMap`.

Closes gh-23215
See gh-23211

spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java

@@ -189,14 +189,13 @@ public abstract class MimeTypeUtils {
 	 * @throws InvalidMimeTypeException if the string cannot be parsed
 	 */
 	public static MimeType parseMimeType(String mimeType) {
-		return cachedMimeTypes.get(mimeType);
-	}
-
-	private static MimeType parseMimeTypeInternal(String mimeType) {
 		if (!StringUtils.hasLength(mimeType)) {
 			throw new InvalidMimeTypeException(mimeType, "'mimeType' must not be empty");
 		}
+		return cachedMimeTypes.get(mimeType);
+	}
 
+	private static MimeType parseMimeTypeInternal(String mimeType) {
 		int index = mimeType.indexOf(';');
 		String fullType = (index >= 0 ? mimeType.substring(0, index) : mimeType).trim();
 		if (fullType.isEmpty()) {

spring-core/src/test/java/org/springframework/util/MimeTypeTests.java

@@ -285,6 +285,12 @@ public class MimeTypeTests {
 				MimeTypeUtils.parseMimeType("audio/*;attr=\""));
 	}
 
+	@Test
+	public void parseMimeTypeNull() {
+		assertThatExceptionOfType(InvalidMimeTypeException.class).isThrownBy(() ->
+				MimeTypeUtils.parseMimeType(null));
+	}
+
 	@Test
 	public void parseMimeTypes() {
 		String s = "text/plain, text/html, text/x-dvi, text/x-c";