Return plausible isReadable() default impl for ext storage

When an ext storage doesn't implement isReadable(), always returning
true made the file scanner believe that the file exists and creates a
cache entry with the size zero.

This fix makes the default impl of isReadable() use file_exists().

Fixes #5940

apps/files_external/lib/streamwrapper.php

@@ -42,11 +42,16 @@ abstract class StreamWrapper extends Common {
 	}
 
 	public function isReadable($path) {
-		return true; //not properly supported
+		// at least check whether it exists
+		// subclasses might want to implement this more thoroughly
+		return $this->file_exists($path);
 	}
 
 	public function isUpdatable($path) {
-		return true; //not properly supported
+		// at least check whether it exists
+		// subclasses might want to implement this more thoroughly
+		// a non-existing file/folder isn't updatable
+		return $this->file_exists($path);
 	}
 
 	public function file_exists($path) {