Ignore empty entries when parsing MediaTypes and MimeTypes

Prior to Spring Framework 5.1.3, MimeTypeUtils.parseMimeTypes() and
MediaType.parseMediaTypes() ignored empty entries, but 5.1.3 introduced
a regression in that an empty entry -- for example, due to a trailing
comma in the list of media types in an HTTP Accept header -- would result
in a "406 Not Acceptable" response status.

This commit fixes this by filtering out empty entries before parsing
them into MimeType and MediaType instances. Empty entries are therefore
effectively ignored.

Fixes gh-23241

spring-web/src/test/java/org/springframework/http/MediaTypeTests.java

@@ -143,6 +143,13 @@ public class MediaTypeTests {
 		assertEquals("Invalid amount of media types", 0, mediaTypes.size());
 	}
 
+	@Test // gh-23241
+	public void parseMediaTypesWithTrailingComma() {
+		List<MediaType> mediaTypes = MediaType.parseMediaTypes("text/plain, text/html, ");
+		assertNotNull("No media types returned", mediaTypes);
+		assertEquals("Incorrect number of media types", 2, mediaTypes.size());
+	}
+
 	@Test
 	public void compareTo() {
 		MediaType audioBasic = new MediaType("audio", "basic");