bedgraphtobigwig with empty bedgraph file, hangs

bedgraphtobigwig with empty bedgraph file, hangs and uses 100% CPU.

```bash
# Create empty bedGraph file.
$ touch empty.bedGraph

# bedgraphtobigwig "hangs" while reading "empty.bedGraph" and uses 100% CPU.
$ bedgraphtobigwig empty.bedGraph hg38.chrom.sizes output.bw
```