Fixed #12161 500 error when accepting assets

[inietov]

Description

When a user tries to accept an assigned asset, if no MAIL_REPLYTO_ADDR environment variable is setted in their .env file, the system shows a 500 error because the return type of the function routeNotificationForMail() is a string.

This PR I checks first if the variable is setted and if it is, returns the address, if don't exists just returns an empty string so the system don't crash.

Fixes #12161

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Test Configuration:

• PHP version: 8.1
• MySQL version: 8.0.23
• Webserver version: nginx/1.19.8
• OS version: Debian 10

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes