Provisioning: Handle empty nested keys on YAML provisioning datasources (#19547)

* Fix: Handle empty nested keys on YAML provisioning datasources

As we provision a datasource via a YAML file, we attempt to transform the
file into sensible Go types that the provisioning code can use.

While this happens, there is a chance some of the keys nested within
the YAML array are empty.

This fix allows the YAML parser to handle empty keys by null checking
the return of `reflect.TypeOf` which according to the documentation:

> TypeOf returns the reflection Type that represents the dynamic type of i. If i is a nil interface value, TypeOf returns nil.

Can return nil.

* Add tests

pkg/services/provisioning/values/values.go

@@ -155,7 +155,13 @@ func (val *StringMapValue) Value() map[string]string {
 // slices and the actual interpolation is done on all simple string values in the structure. It returns a copy of any
 // map or slice value instead of modifying them in place.
 func tranformInterface(i interface{}) interface{} {
-	switch reflect.TypeOf(i).Kind() {
+	typeOf := reflect.TypeOf(i)
+
+	if typeOf == nil {
+		return nil
+	}
+
+	switch typeOf.Kind() {
 	case reflect.Slice:
 		return transformSlice(i.([]interface{}))
 	case reflect.Map: