Bounding box should allow "null" types as value

Description of the issue

The Bounding Box property allows for numeric values. As we generally allow the user to leave fields empty, it should be okay to allow null values. This would also help with simplifying

Steps to Reproduce

1. Create a table or navigate to a table you have permissions for on the OEP
2. Edit the metadata and make sure the Bounding Box property is there in the Extend section of Spatial
3. Leave it empty and attempt to submit the form

Ideas of solution

Add "null" to bounding box items type specification in the schema.json
https://github.com/OpenEnergyPlatform/oemetadata/blob/ed9a205dad73db27487d41ab2918c794830daf22/oemetadata/latest/schema.json#L550

Context and Environment

• Version used:
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• Environment setup and (python) version:

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md