Add null and not null operators

...h/components/criteria/search-type/datalist.component.html

...rch/components/criteria/search-type/datalist.component.ts

...earch/components/criteria/search-type/date.component.html

.../search/components/criteria/search-type/date.component.ts

...h/components/criteria/search-type/datetime.component.html

...rch/components/criteria/search-type/datetime.component.ts

...arch/components/criteria/search-type/field.component.html

...search/components/criteria/search-type/field.component.ts

.../instance/search/components/criteria/search-type/index.ts

...h/components/criteria/search-type/operator.component.html

...omponents/criteria/search-type/operator.component.spec.ts

...rch/components/criteria/search-type/operator.component.ts

...arch/components/criteria/search-type/radio.component.html

...search/components/criteria/search-type/radio.component.ts

...rch/components/criteria/search-type/select.component.html

...earch/components/criteria/search-type/select.component.ts

...earch/components/criteria/search-type/time.component.html

.../search/components/criteria/search-type/time.component.ts

client/src/app/instance/store/models/criterion.model.ts

client/src/app/shared/utils.ts