PYCBC-1681: Raise InvalidArgumentException when base64 vector string is empty

Motivation
==========
Base64 vector strings cannot be empty. The RFC requires us to throw an InvalidArgumentException.

Change
======
If an empty base64 vector string is given, fail with an InvalidArgumentException.

Results
=======
Relevant FIT test passes (VectorSearchBase64Test.base64QueryWithEmptyBase64StringThrowInvalidArgument)

Change-Id: I479bb87b4b326a0c59631816dcc043d25e409b18
Reviewed-on: https://review.couchbase.org/c/couchbase-python-client/+/225921
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Jared Casey <jared.casey@couchbase.com>

Author: DemetrisChr

couchbase/logic/vector_search.py

@@ -127,6 +127,9 @@ def _validate_and_set_vector(self,
         elif not isinstance(vector, str):
             raise InvalidArgumentException('Provided vector must be either a List[float] or base64 encoded str.')
 
+        if len(vector) == 0:
+            raise InvalidArgumentException('Provided base64 encoded vector cannot be empty.')
+
         self._vector_base64 = vector
 
     @classmethod