Fix Assert.instanceOf exception message

Update the exception message used when Assert.instanceOf fails such
that it expects the provided message to end with '.'. This reverts
commit 5874383ef081bb52a872dd49d63e5b542fbf20ca which caused the
implementation to be at odds with the JavaDoc and the previous
release.

The updated code also has the benefit of protecting against a null
message.

Issue: SPR-10269