bin/pdfctl: Fix argument check when 0

.local/bin/pdfctl

@@ -14,7 +14,7 @@ List of commands and their options:
   compress [OPTIONS]
     Compress PDFs.
     -c: Compress raster graphics at the expense of a quality loss.
-    -n:  Do not compress raster graphics at all. Output may be bigger.
+    -n: Do not compress raster graphics at all. Output may be bigger.
 
   extract [-f FIRST-PAGE] [-l LAST-PAGE]
     Extract a range of pages to a single PDF.
@@ -39,6 +39,8 @@ For more details on compression:
 EOF
 }
 
+[ $# -eq 0 ] && usage && exit
+
 cmd="$1"
 shift 1