grid: avoid spurious empty lines in Popen() stdout/log

slapos/grid/utils.py

@@ -109,6 +109,7 @@ class SlapPopen(subprocess.Popen):
       line = self.stdout.readline()
       if line == '' and self.poll() is not None:
         break
+      if line:
         output_lines.append(line)
         logger.info(line.rstrip('\n'))
     self.output = ''.join(output_lines)