[SYNPY-1547] parentWikiId="" Bug

[BWMac]

Description:

A user reported an unhandled error when trying to store a Wiki object in Synapse like so (where syn123 is instead a valid Synapse project ID that the user has access to):

from synapseclient import Wiki
import synapseclient

foo = Wiki(owner="syn123", title="test", parentWikiId="")
syn.store(foo)

This happened because the API does not treat that value of parentWikiId like a None value and instead throws an error. To prevent that error from happening, I added logic in Wiki.__init__ that converts any falsey value that is provided for parentWikiId to None.

Additionally, the user was receiving this error message:

AttributeError: 'SynapseHTTPError' object has no attribute 'message'

rather than a message passed on from the API which I addressed by updating logic here to check the response text.

Testing:
After these changes were made, the script above was able to be run successfully. I also added a unit test to ensure that parentWikiId="" is treated like parentWikiId=None and an integration test to ensure that Wiki objects created with parentWikiId="" are stored successfully in Synapse.

[BryanFauble]

Changes look good logically, just a few minor points