Prevent NullPointerException when User Defined Type is not found.

[boutier]

Bug introduced by:

Introduces @Frozen annotation to mark schema elements as Frozen. (17adf20)

Discussed in #1084

@mp911de: I'm not sure how to test it. Probably something in ColumnTypeResolverUnitTests like the following, but could you please give me how to get a UDT CassandraType? (Or directly write the test)

	@Test // DATACASS-743
	void shouldReportMissingType() {
		CassandraType annotation;

		assertThatThrownBy(() -> resolver.resolve(annotation)).isInstanceOf(MappingException.class);
	}

[pivotal-issuemaster]

@boutier Please sign the Contributor License Agreement!

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[boutier]

I'm not going to sign CLAs for pull requests: it is nonsense since a pull request import all changes from a repository, including licence changes, and, as we can see, there are no licence chances, so the pull request is under the same licence than the original code, i.e. Apache 2 in this case.

Please feel free to do what you deem right: pull from your local repo and then push (eh!), discard pull request, etc.

[mp911de]

The CLA ensures that you grant us an inbound license to import your changes. In this case, we don't require a CLA because it's an obvious fix.

[mp911de]

@pivotal-issuemaster this is an obvious fix.

[pivotal-issuemaster]

@boutier This Pull Request contains an obvious fix. Signing the Contributor License Agreement is not necessary.

[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.