Fixed schema support issue

[vellengs]

schema.properties can be empty. for example:

      "dateArrayRange": {
            "type": "array",
            "items": {
                "type": "string"
            },
            "title": "日期范围",
            "default": [
                "2018-01-02",
                "2018-02-03"
            ],
            "widget": {
                "id": "date-range",
                "format": "YYYY-MM-DD"
            }
        }

the schema is only part of

 {
                "type": "string"
 }

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/cipchk/nz-schema-form/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information