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/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";