fix for null issue during document create event

[manishkumar-dsi]

• Added null & undefined type check in form.js

What does this PR do?

Fix for the defect. It does the null check on value for array type field.

Test Plan

Pre-requisite:

• Go to 'Database' page and create a new 'Collection'
• Create attributes, here at least one attribute should be marked 'Array' as required & 'Required' as optional.
• Once 'Collection' is created.

Test case - 1:

1. Go to Create a new 'Document' page
2. Click on 'Add Attribute' for 'Array' attribute minimum 2 times
3. Do not enter anything in the Value field
4. Create the Document and the document should be created with empty values

Test case - 2:

1. Go to Create a new 'Document' page
2. Click on 'Add Attribute' for 'Array' attribute 1 time
3. Do not enter anything in the Value field
4. Create the Document and the document should be created with empty values

Test case - 3:

1. Select any existing document
2. Update data with different values.
3. document should be updated

Related PRs and Issues

https://github.com/appwrite/appwrite/issues/3528

Have you read the Contributing Guidelines on issues?

Yes