Don't swallow log messages that are null but have a throwable

In some places, I log using:

    Timber.e(throwable, throwable.getMessage())

If the throwable in question was created with a null or empty message, the log message would disappear into the ether. This pull request causes the stack trace to be logged with an empty message instead.
I'm tempted to throw a `IllegalArgumentException` for this... I guess it was already being ignored so might as well leave it.