ColorPicker - form id and empty value support

[VincentRoth]

What problem does this feature solve?

In form, depending of the data model, the color cannot be optionnal and link with the form label is missing.
A default value of #1677ff seems to always be set while we may want to keep an empty (null, undefined...) value.

What does the proposed API look like?

<nz-form-item>
  <nz-form-label nzFor="primaryColor">Primary color</nz-form-label>
  <nz-form-control [nzErrorTip]="error">
    <nz-color-picker nzId="primaryColor" formControlName="primaryColor" />
  </nz-form-control>
</nz-form-item>
```<!-- generated by ng-zorro-issue-helper. DO NOT REMOVE -->