Add support for empty strings in non-key dynamo attributes (#3467)

* Add support for empty strings in non-key attributes

https://github.com/spulec/moto/issues/3339

* Nose, not pytest

* Revert "Nose, not pytest"

This reverts commit 5a3cf6c887dd9fafa49096c82cfa3a3b7f91d224.

* PUT is default action

tests/test_dynamodb2/conftest.py

@@ -0,0 +1,13 @@
+import pytest
+from moto.dynamodb2.models import Table
+
+
+@pytest.fixture
+def table():
+    return Table(
+        "Forums",
+        schema=[
+            {"KeyType": "HASH", "AttributeName": "forum_name"},
+            {"KeyType": "RANGE", "AttributeName": "subject"},
+        ],
+    )