Preserve non-empty PROJECT_FILES environment variable

[claudiu-cristea]

The Issue

• The PR contains the issue scope

In some projects developers might want to control themselves the list of root files and directories that are symlinked. Right now, the symlink-project command overrides any pre-existing environment variable.

How This PR Solves The Issue

Only compute and export PROJECT_FILES if a non-empty value doesn't already exist.

Manual Testing Instructions

1. Create a file .ddev/.env wit the following content:

   PROJECT_FILES="
   my_module.info.yml
   my_module.install
   my_module.module
   src
   "
   

2. Restart DDEV: ddev restart
3. Run ddev symlink-project
4. Check the directory web/modules/custom/my_module for the created symlinks

Automated Testing Overview

N/A

Release/Deployment Notes

N/A