Allow code_location to have no key prefix

[humanzz]

Optionally, enable matching the behaviour when using sagemaker's default bucket where a key prefix is not permitted

Issue #, if available: 226

Description of changes: Addressing item 2.a on the issue

Merge Checklist

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• [ x] I have read the CONTRIBUTING doc
• [ x] I have added tests that prove my fix is effective or that my feature works (if appropriate)
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[codecov-io]

Codecov Report

Merging #227 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #227   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files          45       45           
  Lines        3165     3165           
=======================================
  Hits         2895     2895           
  Misses        270      270

Impacted Files	Coverage Δ
src/sagemaker/estimator.py	85.65% <100%> (ø)	⬆️

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[jesterhazy]

Thanks for the contribution!