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

@@ -277,6 +277,13 @@ public class MimeTypeTests {
 		assertEquals("Invalid amount of mime types", 0, mimeTypes.size());
 	}
 
+	@Test // gh-23241
+	public void parseMimeTypesWithTrailingComma() {
+		List<MimeType> mimeTypes = MimeTypeUtils.parseMimeTypes("text/plain, text/html,");
+		assertNotNull("No mime types returned", mimeTypes);
+		assertEquals("Incorrect number of mime types", 2, mimeTypes.size());
+	}
+
 	@Test // SPR-17459
 	public void parseMimeTypesWithQuotedParameters() {
 		testWithQuotedParameters("foo/bar;param=\",\"");