Beware that getContent() may return null

Beware that getContent() may return null

Before the introduction of the content handler, missing content was
signified by getText() returning null instead of a string. null will
work much like an empty string in most contexts, so in many places,
it was not checked explcitely whether the conent was null.

Now, when getContent() returns null, this often caused a fatal error,
because the code would access whatever getContent() returned as an object,
without checking whether it was null (because no such check was performed
previously, when the content was represented as a string).

This check introduces explicite checks for getContent() returning null
in the most essential core classes.

Change-Id: I551a90b0b67b8edc7570ca5d252ecc1de903f097