Use .get() on REQUEST_ID_UNIQUE_VALUE_PREFIX

[f3ndot]

Direct access to the Flask config when no REQUEST_ID_UNIQUE_VALUE_PREFIX is explicitly set generates a KeyError. Without this pull request you need to do app.config['REQUEST_ID_UNIQUE_VALUE_PREFIX'] = None in order to make the middleware work

...
  File "/usr/src/app/SON/frontend/views.py", line 156, in <module>
    RequestID(app)
  File "/opt/venv/lib/python3.7/site-packages/flask_request_id_header/middleware/__init__.py", line 37, in __init__
    self._unique_request_id_value_prefix = app.config['REQUEST_ID_UNIQUE_VALUE_PREFIX']
KeyError: 'REQUEST_ID_UNIQUE_VALUE_PREFIX'

[kajbaf]

'REQUEST_ID_UNIQUE_VALUE_PREFIX' is supposed to be optional. This PR solves the bug which crashes the system if a value prefix is not provided.