Revise empty MR state text for tone

What does this MR do?

Based on #291998 (closed) from @pedroms, attempts to improve the text suggested to end users who create empty merge requests, to focus on the user experience. I saw text matches in the following places:

• app/views/shared/empty_states/_merge_requests.html.haml I dod NOT update this file. While it matched part of the text I was looking for, it didn't match all of it. I'm not certain if our intent is to change language here.
• app/assets/javascripts/vue_merge_request_widget/components/states/nothing_to_merge.vue I updated this file, because it contained all the text I expected to find.

Questions before taking this work further: should we introduce links into our documentation here? My gut says yes, but doing so is outside my capacity. If the FE engineer tackles the work, I'd suggest two links:

1. "push a new commit" → TBD
2. "use a different branch" → TBD

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #291998 (closed)