Don't append space delimiter when context string is empty.

Summary:
Currently when context string is "", we will still append whitespace to log header. This looks like:
```
I0729 17:36:53.249270 438847 ThriftServerEventHandler.cpp:21 ] Sampling servers
```
which is less readable compared to
```
I0729 17:36:53.249270 438847 ThriftServerEventHandler.cpp:21] Sampling servers
```
. Ideally empty context string should mean "there's no context string to report here", so we should not put an extra delimiter in such case.

Reviewed By: simpkins

Differential Revision: D22835634

fbshipit-source-id: c7fc6de4036050eac3b7d16bf607c637d1394c44

folly/logging/LoggerDB.cpp

@@ -662,9 +662,12 @@ std::string LoggerDB::ContextCallbackList::getContextString() const {
 
   std::string ret;
   callbacks->forEach([&](const auto& callback) {
-    ret += ' ';
     try {
-      ret += callback();
+      auto ctx = callback();
+      if (ctx.empty()) {
+        return;
+      }
+      folly::toAppend(' ', std::move(ctx), &ret);
     } catch (const std::exception& e) {
       folly::toAppend("[error:", folly::exceptionStr(e), "]", &ret);
     };