Python client send a 'null' schema data for StringSchema

[sijie]

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Describe the bug

Python client uses json.dump(None) to generate the schema info for StringSchema. It causes the python client sends a schema info with "null" schema data.

Expected behavior

Python client should send empty schema data for primitive schema types.

[sijie]

The problem was identified when working on apache/pulsar#4360

[tisonkun]

@BewareMyPower I don't know whether it's still relevant so you can triage it and close if invalid anymore.

[BewareMyPower]

We can keep this issue open while I didn't look into this issue.

[erichare]

If i'm understanding this correctly, the problem is in the following lines of schema.py:

class StringSchema(Schema):
    def __init__(self):
        super(StringSchema, self).__init__(str, _pulsar.SchemaType.STRING, None, 'STRING')

Rather than None, would it be more appropriate to send an empty string?

[BewareMyPower]

@erichare I think we need a test to reproduce this bug because currently all tests passed, i.e. this bug is not covered.