[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 [FEATURE]Add isEmpty ppl command #512)
• register a custom expression IsBlankExpression

Do you have any additional context?

• [META]Add Missing PPL commands - campaign #408

[salyh]

We need a final decision wether this should be a command or a function like asked in #512 (comment)

[salyh]

@YANG-DB Is isBlank the same as isEmpty (#512)?