feat(alerting): add nop uploader

ref #6183

pkg/components/imguploader/imguploader.go

@@ -10,6 +10,13 @@ type ImageUploader interface {
 	Upload(path string) (string, error)
 }
 
+type NopImageUploader struct {
+}
+
+func (NopImageUploader) Upload(path string) (string, error) {
+	return "", nil
+}
+
 func NewImageUploader() (ImageUploader, error) {
 
 	switch setting.ImageUploadProvider {
@@ -53,5 +60,5 @@ func NewImageUploader() (ImageUploader, error) {
 		return NewWebdavImageUploader(url, username, password)
 	}
 
-	return nil, fmt.Errorf("could not find specified provider")
+	return NopImageUploader{}, nil
 }