Potential NPE in getVersionSelectedFromRange when the range is null

## Summary
`getVersionSelectedFromRange(...)` dereferences `constraint.getRange().toString()` whenever the constraint has no direct version, without guarding against a null range. A `VersionConstraint` where both `getVersion()` and `getRange()` are null would throw a `NullPointerException`.

## Affected code
- `src/main/java/org/apache/maven/shared/dependency/graph/internal/DefaultDependencyCollectorBuilder.java:238-244`
- `src/main/java/org/apache/maven/shared/dependency/graph/internal/DefaultDependencyGraphBuilder.java:166-172`

## Impact
In practice Aether usually guarantees at least one of version/range is set, so this is an edge case — but it is unguarded and cheap to make robust.

## Suggested fix
```
if ((constraint == null) || (constraint.getVersion() != null) || (constraint.getRange() == null)) {
return null;
}
return constraint.getRange().toString();
```

Read getVersionSelectedFromRange(...) in DefaultDependencyCollectorBuilder.java:238-244 and DefaultDependencyGraphBuilder.java:166-172, comparing how each handles a VersionConstraint without a direct version. Confirm the null-range case is safe in both entry points and that existing behavior for normal version or range constraints is unchanged.

Written by the indexing model from the issue text.