slapos.cookbook: Check if pid file is not empty

  If pidfile is empty, the pgrep will fail forever until the pidfile
  is removed, so check if it exist and it is not empty.

slapos/recipe/librecipe/generic.py

@@ -156,10 +156,12 @@ class GenericBaseRecipe(object):
       lines.append(dedent("""
           # Check for other instances
           pidfile=%s
+          if [ -s $pidfile ]; then
             if pid=`pgrep -F $pidfile -f "$COMMAND" 2>/dev/null`; then
               echo "Already running with pid $pid."
               exit 1
             fi
+          fi
           echo $$ > $pidfile""" % shlex.quote(pidfile)))
 
     lines.append(dedent('''