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

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly