Fix: accept empty schema

[A-Costa]

Closes #88

• Fixes bugs in the code that caused KeyError exceptions when empty Schema is received
• Implement handling of empty schema fields as Variant datatype in Snowflake

[koszti]

thanks for sending this PR. This is a small change but changing a core functionality hence it requires unit and integration tests as well.

Can you please add these tests? Please find some hints below:

1. Unit tests: Add something into test_db_sync.py:test_flatten_schema where no type key in properties but it's an empty dict {}.

2. Integration tests: You can get some inspiration from test_target_snowflake.py:test_nested_schema_flattening . We need to have a new test function with a new test json with empty schema types. The test should load the data from the json with empty schema type {}, and selecting the target table should return VARIANT columns.

Also I'm not sure how we should deal with anyOf? How we can decide which datatype is the good one for snowflake when creating the target table?

The expected behaviour for anyOf should also be covered by test cases.