readline: error on falsy values for callback

It was intended, according to in-test comments and common behaviour,
that callbacks be either `undefined` or a function, but falsy values
were being accepted as meaning "no callback".

PR-URL: https://github.com/nodejs/node/pull/28109
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/readline.js

@@ -121,7 +121,7 @@ function Interface(input, output, completer, terminal) {
     input = input.input;
   }
 
-  if (completer && typeof completer !== 'function') {
+  if (completer !== undefined && typeof completer !== 'function') {
     throw new ERR_INVALID_OPT_VALUE('completer', completer);
   }
 

test/parallel/test-readline-interface.js

@@ -366,6 +366,26 @@ function isWarned(emitter) {
       type: TypeError,
       code: 'ERR_INVALID_OPT_VALUE'
     });
+
+    common.expectsError(function() {
+      readline.createInterface({
+        input: fi,
+        completer: ''
+      });
+    }, {
+      type: TypeError,
+      code: 'ERR_INVALID_OPT_VALUE'
+    });
+
+    common.expectsError(function() {
+      readline.createInterface({
+        input: fi,
+        completer: false
+      });
+    }, {
+      type: TypeError,
+      code: 'ERR_INVALID_OPT_VALUE'
+    });
   }
 
   // Constructor throws if historySize is not a positive number