Fix assertions (#13)

Fixes assertions to allow for calls to write write zero bytes of data.

Author: jabdoa2

serial_asyncio/__init__.py

@@ -118,7 +118,7 @@ def write(self, data):
                 return
             if n == len(data):
                 return  # Whole request satisfied
-            assert n > 0
+            assert n >= 0 < len(data)
             data = data[n:]
             self._ensure_writer()
 
@@ -259,7 +259,7 @@ def _write_ready(self):
                     self._close()
                 return
 
-            assert n > 0
+            assert n >= 0 < len(data)
             data = data[n:]
             self._write_buffer.append(data)  # Try again later
             self._maybe_resume_protocol()