Add option to allow empty parameter value

[tonyjiangh]

The problem to tackle

The default value of parameter.
We have this page about default value in knowledge base.
But since we currently don't allow null value, there has to be some value filled in first as an indicator of using default value.

I think using null for that is just more intuitive.

The fix here

Add an 'Allow empty' option to parameter setting

Functionality

Query like this

select case when '{{val}}' = '' then current_date else '{{val}}' end as value

Could work without preset value for parameter now

New challenge

Empty value handling might be difficult and generate a lot of syntax error. We might want a better error message to help editor get the query right.
Also, if this is merged, I'll try to update the website for new example of usage.

[arikfr]

This needs to be updated with recent changes to parameters implementation. Also, we would like to properly support optional parameters, as described in #3011.

Considering the above, I'm closing this pull request. Thank you for you effort though!