Special-case greetings with an empty greetee.

This fixes a bug where with an empty greetee you would get an extra
space between “hello” and the exclamation mark.

Change-Id: Ifea66e4ac465fdf1ca94e754296d6ef469e062ec
diff --git a/mod/greeting.cpp b/mod/greeting.cpp
index a8101b8..62ebb2d 100644
--- a/mod/greeting.cpp
+++ b/mod/greeting.cpp
@@ -5,9 +5,13 @@
 namespace greeting {
 
 std::string make_greeting(std::string const& greetee) {
-  std::ostringstream out;
-  out << "Hello " << greetee << "!";
-  return out.str();
+  if (greetee == "") {
+    return "Hello!";
+  } else {
+    std::ostringstream out;
+    out << "Hello " << greetee << "!";
+    return out.str();
+  }
 }
 
 }