[Fix] Refactor env() so that when PATH doesn't exsist a NullPointerException isn't thrown

[jonhopper-dataengineers]

What changes are proposed in this pull request?

Modified the UserAgent.env() so that it doesn't throw an exception if the PATH variable isn't available.
relates to issue #550

env is not always available environments which implement sandboxes such as snowflake

How is this tested?

Added additional unit test to validate new behaviour

Describe any tests you have done; especially if test tests are not part of
the unit tests (e.g. local tests).

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Inputs:

• PR number: 633
• Commit SHA: 45b9f783eb5773427546206ac659794aab90cfeb

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