dma-buf: handle empty dma_fence_arrays gracefully

A bug inside the new sync-file merge code created empty dma_fence_array instances.

Warn about that and handle those without crashing.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220329070001.134180-2-christian.koenig@amd.com