Display title in nav only if PLATFORM_NAME is not set

[jazairi]

Why these changes are being introduced:

The app name is inserted into the header based on the PLATFORM_NAME environment variable. If this variable is set, the title should not display

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/GDT-124

How this addresses that need:

This makes the title in the nav render if PLATFORM_NAME is not set.

Side effects of this change:

We will need to make this change downstream, as most (if not all) of our Rails properties have customized the site nav partial.

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[coveralls]

Pull Request Test Coverage Report for Build 7846364563

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.372%

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Totals
Change from base Build 7845201893:	0.0%
Covered Lines:	38
Relevant Lines:	43

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💛 - Coveralls