Fix staticcheck SA5011 nil pointer dereference in retriever test (#2050)

Combine two separate nil checks into a single condition to eliminate
staticcheck SA5011 warnings about possible nil pointer dereference.

The previous code had two sequential checks:
1. if testGroupName == "" { t.Skip(...) }
2. if group == nil { t.Skip(...) }

Staticcheck couldn't determine that t.Skip() exits the test, so it
flagged the later dereference of group.Servers as potentially unsafe.

By combining these into a single condition (testGroupName == "" ||
group == nil), the linter can clearly see that group cannot be nil
when execution continues past the check.

This maintains the same test behavior while satisfying the linter.

Signed-off-by: Juan Antonio Osorio <ozz@stacklok.com>

Author: JAORMX

pkg/runner/retriever/retriever_test.go

@@ -33,12 +33,9 @@ func TestGetMCPServer_WithGroup(t *testing.T) {
 		}
 	}
 
-	if testGroupName == "" {
+	if testGroupName == "" || group == nil {
 		t.Skip("No groups found in registry, skipping group tests")
 	}
-	if group == nil {
-		t.Skip("Test group is nil, skipping")
-	}
 
 	// Find a server in the group to test with
 	var testServerName string