Remove warning when env vars default to blank strings in docker-compose

[sanders41]

Closes #1434

Code Changes

• Changed :- to - when defaulting to an empty string.

Steps to Confirm

• Run nox -s dev
• Verify the following warnings are not present

WARN[0000] The "VAULT_ADDR" variable is not set. Defaulting to a blank string.                                                                                                                                                                                                            
WARN[0000] The "VAULT_TOKEN" variable is not set. Defaulting to a blank string. 
WARN[0000] The "FIDES__CLI__ANALYTICS_ID" variable is not set. Defaulting to a blank string. 
WARN[0000] The "VAULT_NAMESPACE" variable is not set. Defaulting to a blank string. 
WARN[0000] The "FIDES__CLI__ANALYTICS_ID" variable is not set. Defaulting to a blank string. 

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation:
  • documentation complete, PR opened in fidesdocs
  • documentation issue created in fidesdocs
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

Write some things here about the changes and any potential caveats

[eastandwestwind]

lgtm. Confirmed that the warnings were not present when running nox -s dev