Pool amplicon libraries allows for 0 concentration triggering an exception

It's possible to select the down arrow when specifying a concentration. This results in the following exception:

```bash
AN ERROR HAS OCCURED!
Please copy the following into an issue in our github repository

Error
'<' not supported between instances of 'NoneType' and 'float'

Traceback
Traceback (most recent call last):
File "/Users/dtmcdonald/miniconda3/envs/labcontrol/lib/python3.7/site-packages/tornado/web.py", line 1697, in _execute result = method(*self.path_args, **self.path_kwargs)
File "/Users/dtmcdonald/miniconda3/envs/labcontrol/lib/python3.7/site-packages/tornado/web.py", line 3174, in wrapper return method(self, *args, **kwargs)
File "/Users/dtmcdonald/ResearchWork/software/labman/labcontrol/gui/handlers/process_handlers/pooling_process.py", line 310, in post {"function": "amplicon_pool", "parameters": {}})
File "/Users/dtmcdonald/ResearchWork/software/labman/labcontrol/db/process.py", line 2406, in create if in_comp['input_volume'] < 0.001:
TypeError: '<' not supported between instances of 'NoneType' and 'float'
```