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_test.go

@@ -131,6 +131,8 @@ func TestValues(t *testing.T) {
                      - two
                      - three:
                          inside: $STRING
+                     - six:
+                         empty:
                    four:
                      nested:
                        onemore: $INT
@@ -146,11 +148,18 @@ func TestValues(t *testing.T) {
 					"one": 1,
 					"two": "test",
 					"three": []interface{}{
-						1, "two", anyMap{
+						1,
+						"two",
+						anyMap{
 							"three": anyMap{
 								"inside": "test",
 							},
 						},
+						anyMap{
+							"six": anyMap{
+								"empty": interface{}(nil),
+							},
+						},
 					},
 					"four": anyMap{
 						"nested": anyMap{
@@ -166,11 +175,18 @@ func TestValues(t *testing.T) {
 					"one": 1,
 					"two": "$STRING",
 					"three": []interface{}{
-						1, "two", anyMap{
+						1,
+						"two",
+						anyMap{
 							"three": anyMap{
 								"inside": "$STRING",
 							},
 						},
+						anyMap{
+							"six": anyMap{
+								"empty": interface{}(nil),
+							},
+						},
 					},
 					"four": anyMap{
 						"nested": anyMap{