Defend against NPE when no pom is registered using the pom task prior to using the deploy task (fix for #138)

[perNyfelt]

This PR fixes the #138 bug by adding a defensive action avoiding the NPE and instead throws a Build Exception with a clear error message that the user needs to register a pom using the pom task. The fix follows the suggestion of the bug reported (Dirk Schnelle-Walka).

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