settings: clarify that null defaults are supported, using the syntax "${VAR_NAME}"

Using "${VAR_NAME:null}", instead, would define the literal string "null".

settings.json.docker

@@ -65,6 +65,9 @@
  *    "password": "${PASSW}"  // if PASSW is not defined would result in password === null
  *    "password": "${PASSW:}" // if PASSW is not defined would result in password === ''
  *
+ *    If you want to use an empty value (null) as default value for a variable,
+ *    simply do not set it, without putting any colons: "${ABIWORD}".
+ *
  * 3) if you want to use newlines in the default value of a string parameter,
  *    use "\n" as usual.
  *

settings.json.template

@@ -56,6 +56,9 @@
  *    "password": "${PASSW}"  // if PASSW is not defined would result in password === null
  *    "password": "${PASSW:}" // if PASSW is not defined would result in password === ''
  *
+ *    If you want to use an empty value (null) as default value for a variable,
+ *    simply do not set it, without putting any colons: "${ABIWORD}".
+ *
  * 3) if you want to use newlines in the default value of a string parameter,
  *    use "\n" as usual.
  *

src/node/utils/Settings.js

@@ -421,6 +421,14 @@ function storeSettings(settingsObj) {
 /*
  * If stringValue is a numeric string, or its value is "true" or "false", coerce
  * them to appropriate JS types. Otherwise return stringValue as-is.
+ *
+ * Please note that this function is used for converting types for default
+ * values in the settings file (for example: "${PORT:9001}"), and that there is
+ * no coercition for "null" values.
+ *
+ * If the user wants a variable to be null by default, he'll have to use the
+ * short syntax "${ABIWORD}", and not "${ABIWORD:null}": the latter would result
+ * in the literal string "null", instead.
  */
 function coerceValue(stringValue) {
     // cooked from https://stackoverflow.com/questions/175739/built-in-way-in-javascript-to-check-if-a-string-is-a-valid-number
@@ -525,7 +533,7 @@ function lookupEnvironmentVariables(obj) {
     const defaultValue = match[3];
 
     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. Please check your configuration and environment settings.`);
+      console.warn(`Environment variable "${envVarName}" does not contain any value for configuration key "${key}", and no default was given. Returning null.`);
 
       /*
        * We have to return null, because if we just returned undefined, the