GetRequirements API call does not return version in case of maven

As per the documentation https://docs.deps.dev/api/v3alpha/#getrequirements , this API should return

```
maven.dependencies[].version: string
The version requirement of the dependency.
```

When calling https://api.deps.dev/v3alpha/systems/maven/packages/org.apache.httpcomponents.client5:httpclient5/versions/5.3.1:requirements , version is always empty:

```
{
"maven": {
"parent": {
"system": "MAVEN",
"name": "org.apache.httpcomponents.client5:httpclient5-parent",
"version": "5.3.1"
},
"dependencies": [
{
"name": "org.apache.httpcomponents.core5:httpcore5",
"version": "",
"classifier": "",
"type": "",
"scope": "",
"optional": "false",
"exclusions": []
},
{
"name": "org.apache.httpcomponents.core5:httpcore5-h2",
"version": "",
"classifier": "",
"type": "",
"scope": "",
"optional": "false",
"exclusions": []
}, ...
```