fix(generate-openapi-clients): fix error on model template generation

[nachogiljaldo]

The model template seems to have a bug where arrays and maps generate a wrong condition to check if the property is set:

// HasLabels returns a boolean if a field has been set.
func (o *PostHMClustersRequest) HasLabels() bool {
	if o != nil && IsNil(o.Labels) {
		return true
	}

That seems to be wrong, because o.Labels is not set if nil, but it returns true.

[nikimanoledaki]

LGTM, the example you shared shows that this change to the template outputs the expected logic for returning a given resource:

-- if o != nil && IsNil(o.Labels) {
++ if o != nil && !IsNil(o.Labels) {