Implement isBlank

[salyh]

Description

Add isBlank as an "alias" to isEmpty

Issues Resolved

#513

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[salyh]

isBlank is implemented exactly like isEmpty because I dont see any differences between those two functions

[YANG-DB]

@salyh
please add the needed documentation in the following locations:

• ppl commands list
• ppl functions list
• ppl examples doc

[salyh]

@salyh please add the needed documentation in the following locations:

* [ppl commands list](https://github.com/opensearch-project/opensearch-spark/tree/main/docs/ppl-lang)

* [ppl functions list](https://github.com/opensearch-project/opensearch-spark/tree/main/docs/ppl-lang/functions)

* [ppl examples doc](https://github.com/opensearch-project/opensearch-spark/blob/main/docs/ppl-lang/PPL-Example-Commands.md)

already done: https://github.com/opensearch-project/opensearch-spark/pull/749/files

Let me know if you miss anything specific