DefaultIOStream: Remove assert on empty count

fwrite() is valid to call with a 0 count, and will simply return 0.
See:
    https://en.cppreference.com/w/cpp/io/c/fwrite
    http://www.cplusplus.com/reference/cstdio/fwrite/

There are code paths where StreamWriter will call Tell(), which calls
Flush(), which calls Write(buffer.data(), 1, buffer.size()). This can
happen when nothing has yet been written to the buffer, so size is 0.

code/Common/DefaultIOStream.cpp

@@ -100,7 +100,6 @@ size_t DefaultIOStream::Write(const void *pvBuffer,
         size_t pCount) {
     ai_assert(nullptr != pvBuffer);
     ai_assert(0 != pSize);
-    ai_assert(0 != pCount);
 
     return (mFile ? ::fwrite(pvBuffer, pSize, pCount, mFile) : 0);
 }