fix: be defensive about pulling both email and external_id from braze…

[iloveagent57]

… export.
Apparently the export-users endpoint will export records with no "email" in them? 🤷

[iloveagent57]

tests and quality pass in devstack, codecov isn't happy, just gonna merge:

(venv) root@327357d564d1:/edx/src/braze-client# make quality
touch tests/__init__.py
pylint braze tests test_utils *.py
pycodestyle braze tests  *.py
isort --check-only --diff --recursive tests test_utils braze *.py
/edx/src/braze-client/venv/lib/python3.8/site-packages/isort/main.py:1233: UserWarning: W0501: The following deprecated CLI flags were used and ignored: --recursive!
  warn(
/edx/src/braze-client/venv/lib/python3.8/site-packages/isort/main.py:1237: UserWarning: W0500: Please see the 5.0.0 Upgrade guide: https://pycqa.github.io/isort/docs/upgrade_guides/5.0.0.html
  warn(
(venv) root@327357d564d1:/edx/src/braze-client# make test
find . -name '__pycache__' -exec rm -rf {} +
find . -name '*.pyc' -exec rm -f {} +
find . -name '*.pyo' -exec rm -f {} +
find . -name '*~' -exec rm -f {} +
coverage erase
rm -fr build/
rm -fr dist/
rm -fr *.egg-info
pytest
================================================================================== test session starts ===================================================================================
platform linux -- Python 3.8.10, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: /edx/src/braze-client, configfile: tox.ini
plugins: cov-3.0.0
collected 39 items

tests/braze/test_client.py .......................................                                                                                                                 [100%]

---------- coverage: platform linux, python 3.8.10-final-0 -----------
Name                  Stmts   Miss  Cover   Missing
---------------------------------------------------
braze/__init__.py         1      0   100%
braze/client.py         182      3    98%   252, 352, 354
braze/constants.py       18      0   100%
braze/exceptions.py      11      0   100%
---------------------------------------------------
TOTAL                   212      3    99%
Coverage XML written to file coverage.xml


=================================================================================== 39 passed in 2.60s ===================================================================================