[FEATURE]Add `isBlank` ppl command

[YANG-DB]

Is your feature request related to a problem? Add isBlank command for PPL spark based execution driver .

Since spark sql doesnt offer such expression a possible option would be:

• translate the expression into a where clause ( similar to #512)
• register a custom expression IsBlankExpression

Do you have any additional context?

• https://github.com/opensearch-project/opensearch-spark/issues/408

[salyh]

We need a final decision wether this should be a command or a function like asked in https://github.com/opensearch-project/opensearch-spark/issues/512#issuecomment-2331451920