missing default profile causes assume to fail

[zaro0508]

assume fails if you don't have a default profile in ~/.aws/credentials file. Even worse than that assume will wipe out the contents of the credentials file in this scenario.

steps to repro:

1. setup ~/.aws/credentials and ~/.aws/config files. Make sure [default] section is NOT in the credentials file.
2. run assume add command.
   Note: fails with Error no section default and now the credentials file is empty.

➜  python -V
Python 3.7.9

➜  assume --version
0.4.0

➜  assume add myrole --profile testitorgmgnt-servicerole --role-arn arn:aws:iam::XXXXXXXXX:role/MyAccountAccessRole
Added role 'myrole' to the configuration. You can now assume this role with:

assume switch myrole

➜  assume list
myrole:
  profile: testitorgmgnt-servicerole
  role_arn: arn:aws:iam::XXXXXXXXX:role/MyAccountAccessRole

➜  assume switch myrole
Traceback (most recent call last):
  File "/Users/zaro0508/.virtualenvs/py37-default/bin/assume", line 8, in <module>
    sys.exit(main())
  File "/Users/zaro0508/.virtualenvs/py37-default/lib/python3.7/site-packages/aws_assume_role_helper/__main__.py", line 23, in main
    print(commands.switch(arguments.name))
  File "/Users/zaro0508/.virtualenvs/py37-default/lib/python3.7/site-packages/aws_assume_role_helper/commands.py", line 91, in switch
    for key in config_credentials.options('default'):
  File "/usr/local/var/pyenv/versions/3.7.9/lib/python3.7/configparser.py", line 674, in options
    raise NoSectionError(section) from None
configparser.NoSectionError: No section: 'default'