base plugin: do not decode the empty string

If the json was empty, for example if the qemu-img command times out, a
message

    warn "could not parse qemu-img info command output for '$filename' - $err\n";

would have been printed.

This message could lead one to think the issue lies in the contents of
the json, even if the previous warning said that there was a timeout.

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>

src/PVE/Storage/Plugin.pm

@@ -974,6 +974,10 @@ sub file_size_info {
 	# otherwise we warn about it and try to parse the json
 	warn $err_output;
     }
+    if (!$json) {
+	# skip decoding if there was no output, e.g. if there was a timeout.
+	return wantarray ? (undef, undef, undef, undef, $st->ctime) : undef;
+    }
 
     my $info = eval { decode_json($json) };
     if (my $err = $@) {