fix: loading empty content (#2237)

* content may be a empty string but !"" = true so it will use
`ComponentLoader`, but we should load it as it has content
(empty content)

Author: oldshensheep

src/files/file-preview/FilePreview.js

@@ -113,7 +113,7 @@ const Preview = (props) => {
         return cantPreview
       }
 
-      if (!content) {
+      if (content === null) {
         return <ComponentLoader />
       }