Handle null multichoice value

[jwalgran]

There is some data in the production database where the value of a multichoice field audit is the string "null". Adding a None check to the clean_value function prevents an exception where code further down in the function expects the value to be iterable.

---

Testing

/greenprintmaps/features/2081911/

The production data has an audit that looks like this

otm=# select field, previous_value, current_value  from treemap_audit where model = 'Tree' and model_id = 1814485 and previous_value = 'null' order by created;
        field         | previous_value | current_value
----------------------+----------------+---------------
 udf:Name of Project: | null           | null

On the develop branch, loading the detail page for this plot crashes. On the PR branch, it loads properly.

---

Connects to https://github.com/OpenTreeMap/otm-addons/issues/1526

[coveralls]

Coverage decreased (-0.003%) to 83.707% when pulling c3a8683 on jwalgran:jcw/handle-null-multichoice-value into 92495b7 on OpenTreeMap:develop.

[jwalgran]

@azavea-bot rebuild

[jwalgran]

@azavea-bot rebuild

[jwalgran]

@azavea-bot rebuild

[jwalgran]

@azavea-bot rebuild

[jwalgran]

(I am working through a problem with provisioning the PR builder VM)