Settings: Deprecate null as the default default value

CHANGELOG.md

@@ -23,6 +23,9 @@
     `"${UNSET_VAR:undefined}"`), the setting is now removed instead of set to
     the string "undefined". It is no longer possible to produce the string
     "undefined" via environment variable substitution.
+  * Support for unset variables without a default value is now deprecated.
+    Please change all instances of `"${FOO}"` in your `settings.json` to
+    `${FOO:null}` to keep the current behavior.
 
 ### Notable enhancements
 

settings.json.docker

@@ -45,7 +45,9 @@
  *       - "${UNSET_VAR:undefined}" causes the setting to be removed (or be set
  *         to null, if used as a member of an array).
  *   - If the environment variable is unset and no default value is provided,
- *     the value becomes null.
+ *     the value becomes null. THIS BEHAVIOR MAY CHANGE IN A FUTURE VERSION OF
+ *     ETHERPAD; if you want the default value to be null, you should explicitly
+ *     specify "null" as the default value.
  *
  * EXAMPLE:
  *    "port":     "${PORT:9001}"

settings.json.template

@@ -36,7 +36,9 @@
  *       - "${UNSET_VAR:undefined}" causes the setting to be removed (or be set
  *         to null, if used as a member of an array).
  *   - If the environment variable is unset and no default value is provided,
- *     the value becomes null.
+ *     the value becomes null. THIS BEHAVIOR MAY CHANGE IN A FUTURE VERSION OF
+ *     ETHERPAD; if you want the default value to be null, you should explicitly
+ *     specify "null" as the default value.
  *
  * EXAMPLE:
  *    "port":     "${PORT:9001}"

src/node/utils/Settings.js

@@ -599,7 +599,9 @@ const lookupEnvironmentVariables = (obj) => {
 
     if ((envVarValue === undefined) && (defaultValue === undefined)) {
       console.warn(`Environment variable "${envVarName}" does not contain any value for ` +
-                   `configuration key "${key}", and no default was given. Returning null.`);
+                   `configuration key "${key}", and no default was given. Using null. ` +
+                   'THIS BEHAVIOR MAY CHANGE IN A FUTURE VERSION OF ETHERPAD; you should ' +
+                   'explicitly use "null" as the default if you want to continue to use null.');
 
       /*
        * We have to return null, because if we just returned undefined, the