Don't map empty objects as an attribute with value "[object Object]"

Oops. Empty objects were incorrectly being set as attributes with the
value "[object Object]", which would cause updateStyles to crash in
rather spectacular ways when setting the same text alignment multiple
times on a paragraph style that had an empty text properties obj.
Merge pull request #401 from peitschie/merge-obj-fix

Don't map empty objects as an attribute with value "[object Object]"