set-source-date-epoch-to-latest: fix warning

If there was no older file than $NIX_BUILD_TOP this would result in a
warning, e.g. with nix-info.

```
/nix/store/15kgcm8hnd99p7plqzx7p4lcr2jni4df-set-source-date-epoch-to-latest.sh: line 13: [: : integer expression expected
```