fix(code-gen): date validator should support special string cases

[dirkdev98]

The Date validator now passes isOptional and allowNull values down to the string validator. So dates can be set to null or undefined if an empty string is passed in.

Closes #551

[github-actions]

==== Benchmark result ====

Node.js version: 14
Total benchmarks: 15
          Passed: 15
          Failed: 0
-----------
router - github static path              200000  iterations    8003  ns/op
router - github path params              100000  iterations   10480  ns/op
compas validator simple                 5000000  iterations     264  ns/op
yup validator simple                     100000  iterations   12262  ns/op
fastest-validator validator simple     10000000  iterations     114  ns/op
compas validator nested                 1000000  iterations    1442  ns/op
yup validator nested                      50000  iterations   37931  ns/op
fastest-validator validator nested      2000000  iterations     548  ns/op
logger - strings                         500000  iterations    2578  ns/op
logger - objects                         500000  iterations    3242  ns/op
logger - deep objects                    200000  iterations    5859  ns/op
uuid                                    5000000  iterations     490  ns/op
uuid.isValid                           10000000  iterations     163  ns/op
isNil                                 500000000  iterations       0  ns/op
isPlainObject                         100000000  iterations      18  ns/op

[github-actions]

==== C8 test coverage ====
Statements: 66.12% ( 25373/38373 )
Branches : 65.12% ( 2774 /4260 )
Functions : 66.93% ( 834 /1246 )
Lines : 66.12% ( 25373 /38373 )

[github-actions]

==== Benchmark result ====

Node.js version: 15
Total benchmarks: 15
          Passed: 15
          Failed: 0
-----------
router - github static path              200000  iterations    8497  ns/op
router - github path params              100000  iterations   10462  ns/op
compas validator simple                 5000000  iterations     210  ns/op
yup validator simple                     200000  iterations   10270  ns/op
fastest-validator validator simple     20000000  iterations      84  ns/op
compas validator nested                 1000000  iterations    1198  ns/op
yup validator nested                      50000  iterations   37352  ns/op
fastest-validator validator nested      5000000  iterations     449  ns/op
logger - strings                         500000  iterations    2398  ns/op
logger - objects                         500000  iterations    2770  ns/op
logger - deep objects                    500000  iterations    4052  ns/op
uuid                                    5000000  iterations     316  ns/op
uuid.isValid                           10000000  iterations     120  ns/op
isNil                                 500000000  iterations       0  ns/op
isPlainObject                         100000000  iterations      15  ns/op