code-gen: fix field types of update partial for optional fields

[dirkdev98]

Closes #643

[github-actions]

==== Benchmark result ====

Node.js version: 15
Total benchmarks: 15
          Passed: 15
          Failed: 0
-----------
router - github static path              200000  iterations    8935  ns/op
router - github path params              100000  iterations   11325  ns/op
compas validator simple                 5000000  iterations     272  ns/op
yup validator simple                     100000  iterations   13182  ns/op
fastest-validator validator simple     10000000  iterations     109  ns/op
compas validator nested                 1000000  iterations    1549  ns/op
yup validator nested                      20000  iterations   52592  ns/op
fastest-validator validator nested      2000000  iterations     574  ns/op
logger - strings                         500000  iterations    2800  ns/op
logger - objects                         500000  iterations    3423  ns/op
logger - deep objects                    500000  iterations    5654  ns/op
uuid                                    5000000  iterations     390  ns/op
uuid.isValid                           10000000  iterations     153  ns/op
isNil                                 500000000  iterations       0  ns/op
isPlainObject                          50000000  iterations      21  ns/op

[github-actions]

==== Benchmark result ====

Node.js version: 14
Total benchmarks: 15
          Passed: 15
          Failed: 0
-----------
router - github static path              200000  iterations    8192  ns/op
router - github path params              100000  iterations   10032  ns/op
compas validator simple                 5000000  iterations     260  ns/op
yup validator simple                     100000  iterations   12799  ns/op
fastest-validator validator simple     10000000  iterations     124  ns/op
compas validator nested                 1000000  iterations    1386  ns/op
yup validator nested                      50000  iterations   39341  ns/op
fastest-validator validator nested      2000000  iterations     550  ns/op
logger - strings                         500000  iterations    2866  ns/op
logger - objects                         500000  iterations    3383  ns/op
logger - deep objects                    500000  iterations    5854  ns/op
uuid                                    2000000  iterations     508  ns/op
uuid.isValid                           10000000  iterations     172  ns/op
isNil                                 500000000  iterations       0  ns/op
isPlainObject                         100000000  iterations      19  ns/op

[github-actions]

==== C8 test coverage ====
Statements: 66.19% ( 24515/37036 )
Branches : 65.47% ( 2688 /4106 )
Functions : 65.93% ( 811 /1230 )
Lines : 66.19% ( 24515 /37036 )