fix: handle nil jsonpointer sources (#213)

Co-authored-by: Tristan Cartledge <108070248+TristanSpeakEasy@users.noreply.github.com>

Author: kv1sidisi

jsonpointer/jsonpointer.go

@@ -127,6 +127,9 @@ func getTarget(source any, currentPart navigationPart, stack []navigationPart, c
 	}
 
 	sourceType := reflect.TypeOf(source)
+	if sourceType == nil {
+		return nil, nil, ErrNotFound.Wrap(fmt.Errorf("source is nil at %s", currentPath))
+	}
 	sourceElemType := sourceType
 
 	if sourceType.Kind() == reflect.Ptr {

jsonpointer/jsonpointer_test.go

@@ -367,6 +367,14 @@ func TestGetTarget_Error(t *testing.T) {
 			},
 			wantErr: errors.New("not found -- slice is nil at /0"),
 		},
+		{
+			name: "nil source",
+			args: args{
+				source:  nil,
+				pointer: JSONPointer("/key1"),
+			},
+			wantErr: errors.New("not found -- source is nil at /key1"),
+		},
 		{
 			name: "nil struct",
 			args: args{