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'
```


Beginne mit dem Handler für das Pooling-Formular in labcontrol/gui/handlers/process_handlers/pooling_process.py und verfolge die Anfrage in labcontrol/db/process.py um Zeile 2406, wo input_volume mit 0.001 verglichen wird. Reproduziere die Eingabe über den Abwärtspfeil und untersuche, wie die Konzentration zu None wird. Die Aufgabe ist abgeschlossen, wenn der Pfad für die Konzentration null nicht mehr die gemeldete Ausnahme auslöst.

Vom Indexierungsmodell aus dem Issue-Text verfasst.