Unnecessary empty references node created when merging BOMs

The cyclonedx-cli tool creates an empty ```` node inside the ```` node, when no such reference exists in the input data.

How to reproduce:

Example BOM 1 with a patch (example.bom.xml):

```xml

Apache Solr
7.7.2


Apache-2.0


cpe:2.3:a:apache:solr:7.7.2:*:*:*:*:*:*:*





CVE-2021-44228
Log4J Patch
Log4J update to 2.16.0

NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-44228

```

Example 2 (example2.bom.xml):
```xml

Foobar
1.2.3

```

Merge with cyclonedx-cli:
```
cyclonedx-cli --version
0.24.2
cyclonedx-cli merge --input-files example.bom.xml example2.bom.xml --output-format xml
```

Result contains the empty `````` node.
```xml

Apache Solr
7.7.2


Apache-2.0


cpe:2.3:a:apache:solr:7.7.2:*:*:*:*:*:*:*





CVE-2021-44228
Log4J Patch
Log4J update to 2.16.0

NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-44228









Foobar
1.2.3

```