du: env var with a zero block size should be ignored

[cakebaker]

While trying to reproduce #8913, I noticed that we fail if one of the block size environment variables is zero, whereas GNU du uses the default block size in such a case.

$ DU_BLOCK_SIZE=0 cargo run -q du README.md
du: invalid --block-size argument '???BUG'
$ BLOCK_SIZE=0 cargo run -q du README.md
du: invalid --block-size argument '???BUG'
$ BLOCKSIZE=0 cargo run -q du README.md
du: invalid --block-size argument '???BUG'

$ DU_BLOCK_SIZE=0 du README.md
12      README.md
$ BLOCK_SIZE=0 du README.md
12      README.md
$ BLOCKSIZE=0 du README.md
12      README.md
$ du README.md
12      README.md

[Ada-Armstrong]

Fix for this is essentially the same as #8932. Just need to figure out how to test it since test_du has no BLOCK_SIZE env var tests currently.