Add sanity check in regenerating doc pages

[shreya0204]

Fixes: https://github.com/wp-cli/handbook/issues/507

• As per the issue, the generation of internal-api was failing because of the warnings thrown by the following code.
• The warnings originates from an assumption that preg_match would always find a match.
• A sanity check has been added in the parse_docblock method to handle invalid JSON output gracefully.

[ernilambar]

This PR should fix the issue https://github.com/wp-cli/handbook/pull/520