Validation step of generate fails if there are no autogenerated models to generate

### What happened?
Validation step of generate fails if there are no autogenerated models to generate.

Error message when running `gqlgen generate`
```
validation failed: packages.Load: -: no matching versions for query "latest"
```

It looks like if there are no autogenerated models to generate, the file doesn't get created, which makes validation fail. It should instead generate a stub file, so validation will pass.

### What did you expect?
No validation error to occur

### Minimal graphql.schema and models to reproduce
```
model:
filename: graph/model/models_gen.go
package: model
```
(Default from init)

### versions
- `gqlgen version` v0.11.3
- `go version` 1.14.1
- Using go modules