Fix date with empty string and handle invalid date format during execution

[rlaiola]

This PR:

• Highlights invalid date format while typing if an empty string is used as parameter of the date function;

Database Employee (Franklin University)

pi date('')->d EMPLOYEE

Before:

After:

• Shows error message if the date check problem happens during execution;

Database Employee (Franklin University)

pi date(Fname)->d EMPLOYEE

Before (after clicking on the execute query):

After:

PS: I'd suggest reverting the changes in src/db/parser/grammar_bags.d.ts because the problem is not date-related and it will be fixed in dbis-uibk#215

[KPMGE]

@rlaiola Thanks for the points. Great improvements!