Minor improvement: Allow writing 0 bytes to SSL socket.

[Timbus]

Hey,
I was just patching SSL/TLS support into amqp.cr and it failed after calling socket.write() with a 0-length slice argument. The normal IO socket accepts this silently.
The error occurs in unbuffered_write:

crystal/src/openssl/ssl/socket.cr

Lines 107 to 116 in 7fb783f

	def unbuffered_write(slice : Bytes)
	check_open
	count = slice.size
	bytes = LibSSL.ssl_write(@ssl, slice.pointer(count), count)
	unless bytes > 0
	raise OpenSSL::SSL::Error.new(@ssl, bytes, "SSL_write")
	end
	nil
	end

because openssl will return an "unexpected EOF" if count == 0. (It might also just be returning a 0, because 0 bytes are written.. IDK)

It would be nice if the behaviour was aligned with the standard socket, and it just succeeded after "writing" a 0-length buffer.