Fixed IBANFormField to support empty iban

[TomRaz]

Currently, when using IBANField in django admin, and the field is nullable and blank=true, an exception is raised when trying to save the form without an iban. I added handling of this case in the IBANFormField. All Changes

• [V ] Add an entry to the docs/changelog.rst describing the change.

• [ V] Add an entry for your name in the docs/authors.rst file if it's not already there.

• [V ] Adjust your imports to a standard form by running this command:

  isort --recursive --line-width 120 localflavor tests

[codecov-io]

Codecov Report

Merging #297 into master will decrease coverage by 0.04%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
- Coverage   96.01%   95.97%   -0.05%     
==========================================
  Files         150      150              
  Lines        4167     4169       +2     
  Branches      567      568       +1     
==========================================
  Hits         4001     4001              
- Misses        107      108       +1     
- Partials       59       60       +1

Impacted Files	Coverage Δ
localflavor/generic/forms.py	96.29% <0%> (-3.71%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 7b92156...4102603. Read the comment docs.

[benkonrath]

Thanks for pointing out this issue and for the fix. It seems it's a general problem for a lot of the form fields that inherit forms.CharField. I'm working on a complete fix for all of the fields in #298. Let's keep this open until #298 is merged.