storage/stream/flash: flash_sync: Check if there is anything to write

The commit moves block that checks if flash_stream_ctx.buf_bytes
is non-zero out of CONFIG_STREAM_FLASH_ERASE specific code,  as that is
universal check and is valid also for not CONFIG_STREAM_FLASH_ERASE
code.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>