Environment variable overrides are ignored if default setting is empty

[tdonohue]

Describe the bug

If you attempt to use environment variables to override an empty setting, it won't work.

Take the setting rest.ssrBaseUrl as an example:

• This setting is not defined in the default-app-config.ts
• This setting is not specified in the default config.yml
• However, it is valid setting defined in server-config.interface.ts
• Therefore, rest.ssrBaseUrl defaults to an empty value.

If you attempt to specify it via an environment variable, that value will be ignored

DSPACE_REST_SSRBASEURL: http://localhost:8080/server

This issue impacts all settings which are not listed in default-app-config.ts.

One way to fix this issue would be to ensure all settings with empty default values are added to default-app-config.ts. They could be given an empty string value in that file, or some other sane default. However, we'd need to ensure there are no side effects (in other areas of the code) to giving these settings a default value. (I also feel this is a semi-fragile fix, as we'd have to remember to always add every setting to default-app-config.ts)

To Reproduce

Steps to reproduce the behavior:

1. Set the environment variable DSPACE_REST_SSRBASEURL to http://localhost:8080/server as shown above
2. Start up the UI in production mode: npm run build:prod; npm run serve:ssr
3. Notice that the SSR logs will NOT say: Applying environment variable DSPACE_UI_SSRBASEURL with value http://localhost:8080/server
4. Stop the UI
5. Now, modify the default-app-config.ts to add a default value for ssrBaseUrl:

   rest: ServerConfig = {
     ...
     // Just add this line - The value you give it doesn't matter
     ssrBaseUrl: 'http://dspace.org',
   };
   

6. Rebuild the UI and run again in production mode: npm run build:prod; npm run serve:ssr
7. Notice that the SSR Logs will now say: Applying environment variable DSPACE_UI_SSRBASEURL with value http://localhost:8080/server

Expected behavior

It should be possible to use environment variables to give a value to empty settings.

Related work

Discovered while debugging issues with #5276 . This PR was attempting to use an empty setting ui.BaseUrl which could NOT be overridden in Docker Compose scripts until that setting was added to default-app-config.ts with a default value.