Bounding box should allow "null" types as value

[jh-RLI]

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

oemetadata/oemetadata/latest/schema.json

Line 550 in ed9a205

"type": "number"

Context and Environment

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

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md