repl: eval empty lines, fixing debugger repeat

Tweak the better empty line handling introduced in nodejs#2163 so that empty
lines are still passed to the eval function. This is required for the
debugger to repeat the last command on an empty line.

Fixes: nodejs#6010

lib/repl.js

@@ -421,7 +421,7 @@ function REPLServer(prompt,
       }
     }
 
-    if (!skipCatchall && (cmd || (!cmd && self.bufferedCommand))) {
+    if (!skipCatchall) {
       var evalCmd = self.bufferedCommand + cmd;
       if (/^\s*\{/.test(evalCmd) && /\}\s*$/.test(evalCmd)) {
         // It's confusing for `{ a : 1 }` to be interpreted as a block
@@ -483,7 +483,7 @@ function REPLServer(prompt,
           // immediately. We don't have to print anything else. So, only when
           // the second argument to this function is there, print it.
           arguments.length === 2 &&
-          (!self.ignoreUndefined || ret !== undefined)) {
+          (!(self.ignoreUndefined || evalCmd === '\n') || ret !== undefined)) {
         if (!self.underscoreAssigned) {
           self.last = ret;
         }