Don't attempt to read stdin if it is empty.

 * Check for available data size in v1/shell.py/_set_data_field, don't
   read if 0.
 * Add test_shell.py including tests for 3x stdin scenarios:
	* closed
 	* open and empty
	* open with data

Change-Id: I6ff65b0e226be509de9cd3f021560081529283b0
Fixes: bug #1173044