This fixes the bug introduced by f1091a484b.
The problem is that condition was returning string which
could not be converted to boolean. We solve that by showing
`Start Printing` when `is_active`.
```
ERROR:root:Unhandled exception during run
Traceback (most recent call last):
...
File "/opt/klipper/klippy/extras/display/menu.py", line 117, in eval_enable
return bool(ast.literal_eval(self._enable_tpl.render(context)))
File "/usr/lib/python2.7/ast.py", line 49, in literal_eval
node_or_string = parse(node_or_string, mode='eval')
File "/usr/lib/python2.7/ast.py", line 37, in parse
return compile(source, filename, mode, PyCF_ONLY_AST)
File "<unknown>", line 1
/opt/octoprint/uploads/wait_print.gcode
```
Signed-off-by: Kamil Trzcinski <ayufan@ayufan.eu>
7e88f9220c