t:/Pack produces dependencies for content only packages

### Steps to reproduce

Pack this project file:

```

netstandard20
false

```

(You will need to create the foo.txt file in the project directory).

Directory contents:
```
/
- foo.txt
```

Command line
```
msbuild /t:pack
```
### Expected behavior

Nuget package does not have any dependencies.

### Actual behavior
The nuspec file in the produced nuget package expresses the following dependency:
```



```

### Environment data
`msbuild /version` output: 15.5.180.51428

OS info: windows 10