Check for undefined globalValue

This commit changes the check for global values of the frontend env variables from
a non-null assertion to a check for existence (not undefined). When performing a non-null
assertion, in the dockerized verison of the app, empty, defined env vars would be caught as unset.

frontend/src/config.ts

@@ -9,7 +9,7 @@ const getEnvVariable = (name: string): string => {
         // If global value equals its placeholder value 'placeholderValue', it is considered undefined
         const placeholderValue: string = '${' + name + '}'
 
-        if (globalValue === placeholderValue || !globalValue)
+        if (globalValue === placeholderValue || globalValue === undefined)
             throw new Error(
                 `Global variable "${name}" is not set. Verify that your .env file is up to date with .env.example`
             )