Optional folders referenced by dockerfiles causes `skaffold dev` to ignore changes

I believe this is related to #2433

### Expected behavior

We use a tool called [yalc](https://github.com/whitecolor/yalc) in local development to copy npm module changes in our environment without needing to publish to central repository (and avoiding symlink issues with containers). Packages are copied into a `.yalc` folder in the dependent application, that folder is copied in via our dockerfile if it exists:

``` dockerfile
....

ONBUILD COPY ./.npmrc ./.npmrc
ONBUILD COPY ./package.json ./package.json
ONBUILD COPY ./package-lock.json ./package-lock.json

ONBUILD COPY .yalc* .yalc

....
```

We would expect Skaffold to watch for changes in that .yalc folder if it exists, and ignore it otherwise.

### Actual behavior
If that .yalc folder doesn't exist, then we get this error:
`WARN[24575] Ignoring changes: listing files: file pattern [.yalc*] must match at least one file `

Any file changes in that folder are ignored, and builds are not triggered.

### Information

- Skaffold version:v1.13.1