#1291 Updated sum function docs to mention array should not be empty

[cerebro1]

Related issue

Closes #1291.

Proposed Changes

The page does not mention the caveat that the sum function in Kyverno requires at least 1 element in the array, while the JMESPath specification says that the result should be 0 when the array is empty.

##Verify

sonali@SONALI-SRIVASTAVA:~/github/website$ echo '{"input":[]}' | kyverno jp query "sum(input)"
Reading from terminal input.
Enter input object and hit Ctrl+D.
Error: error evaluating JMESPath expression: JMESPath function 'sum': at least one element in the array is required

Checklist

• I have read the contributing guidelines.
• I have inspected the website preview for accuracy.
• I have signed off my issue.

Attached the preview:

[cerebro1]

@realshuting @vishal-chdhry Could you review this, please?

[realshuting]

/lgtm

Thanks @cerebro1 !