Is the reason phrase in a response message optional?

Is the reason phrase truly optional?  That is, can a response message status
line look like the following:

HTTP/1.1<SP>200<CR><LF>

Or, is the reason phrase required but allowed to be empty?  Meaning a
response message without a reason phrase must look like:

HTTP/1.1<SP>200<SP><CR><LF>

Any assistance that you can provide regarding an appropriate interpretation
will be greatly appreciated.

Regards,
David Deschenes
EXTOL International