if no task input, don't proceed to start tracking

Following crash happens when user starts tracking without entering Activity, Tags, or both.

```
Traceback (most recent call last):
  File "/home/chen/git/hamster/src/hamster/today.py", line 446, in on_switch_activity_clicked
    tags = self.new_tags.get_text().decode("utf8", "replace"))
  File "/home/chen/git/hamster/src/hamster/lib/stuff.py", line 271, in __init__
    input_parts = activity.strip().split(" ")
AttributeError: 'NoneType' object has no attribute 'strip'
```