empty files are reported with empty hash in spdx tag/value, that makes it invalid

### Description

The generated spdx in tag/value format is invalid, if the scan contained an empty file, e.g. the following part is invalid since it misses the SHA1 hash:

```
131 │ # File
132 │
133 │ FileName: ./public/.keep
134 │ FileChecksum: SHA1:
135 │ LicenseConcluded: NOASSERTION
136 │ LicenseInfoInFile: NONE
137 │ FileCopyrightText: NONE
```

### How To Reproduce

* scan a directory with an empty file
* generate tag/value
* use https://tools.spdx.org/app/validate/ to validate and see the following violation:


### System configuration
v21.8.4