[BUG] [flytekit][mypy] Fix Incompatible types

[pingsutw]

Describe the bug
Related to #1030
Fix mypy errors like "Incompatible types ..."

plugins/flytekit-greatexpectations/flytekitplugins/great_expectations/schema.py:54: error: Incompatible types in assignment (expression has type "None", variable has type "BatchRequestConfig")
plugins/flytekit-greatexpectations/flytekitplugins/great_expectations/schema.py:70: error: Incompatible return value type (got "Tuple[Type[str], GreatExpectationsFlyteConfig]", expected "Tuple[Type[Any], Type[GreatExpectationsFlyteConfig]]")
plugins/flytekit-greatexpectations/flytekitplugins/great_expectations/schema.py:84: error: Incompatible return value type (got "Tuple[Type[Any], GreatExpectationsFlyteConfig]", expected "Tuple[Type[Any], Type[GreatExpectationsFlyteConfig]]")
plugins/flytekit-greatexpectations/flytekitplugins/great_expectations/schema.py:325: error: Incompatible return value type (got "str", expected "GreatExpectationsType")

Expected behavior
Should pass the mypy checks.

[Optional] Additional context
To Reproduce
Steps to reproduce the behavior:

1. make test | grep "Incompatible"

Screenshots
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