dma-buf/sync-file: fix logic error in new fence merge code

When the array is empty because everything is signaled we can't use
add_fence() to add something because that would filter the signaled
fence again.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 519f490d

 ("dma-buf/sync-file: fix warning about fence containers")
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220329070001.134180-1-christian.koenig@amd.com