Only perform minItems validation when field is provided

[nick-jones]

As reported in #86, minItems validation is being applied even when a non-required field is omitted from the input json.

A json schema like

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "id": "https://example.com/array",
  "type": "object",
  "properties": {
    "myArray": {
      "type": "array",
      "items": {
        "type": "string"
      },
      "minItems": 1,
      "maxItems": 3
    },
  }
}

should pass against an empty object input

{}

(can be tested at https://www.jsonschemavalidator.net/)

Instead the generated code will result in an error during unmarshal. The proposed solution is to just add a nil check - non-required slices use omitempty, which means they end up nil if not supplied. If the field is required, the earlier field presence check will catch these - but also they'll never be nil because omitempty is not used under such circumstances. So overall this additional constraint seems safe to add. Happy to take direction on other approaches.

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (0f951fd) 78.14% compared to head (5f48a22) 78.14%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #122 +/- ## ======================================= Coverage 78.14% 78.14% ======================================= Files 10 10 Lines 1446 1446 ======================================= Hits 1130 1130 Misses 244 244 Partials 72 72 ``` | [Files](https://app.codecov.io/gh/omissis/go-jsonschema/pull/122?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Claudio+Beatrice) | Coverage Δ | | |---|---|---| | [pkg/generator/validator.go](https://app.codecov.io/gh/omissis/go-jsonschema/pull/122?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Claudio+Beatrice#diff-cGtnL2dlbmVyYXRvci92YWxpZGF0b3IuZ28=) | `95.23% <100.00%> (ø)` | |

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