chore(bigquery): relax BI Engine Statistics test (#5084)

Test had an expectation on BI Engine statistics presence, however
test environments don't have BI resources provisioned.  Relax the
test.

Fixes: #5083

bigquery/integration_test.go

@@ -2016,18 +2016,16 @@ func TestIntegration_QueryStatistics(t *testing.T) {
 		t.Error("expected query timeline, none present")
 	}
 
-	if qStats.BIEngineStatistics == nil {
-		t.Error("expected BIEngine statistics, none present")
-	}
-
-	expectedMode := true
-	for _, m := range []string{"FULL", "PARTIAL", "DISABLED"} {
-		if qStats.BIEngineStatistics.BIEngineMode == m {
-			expectedMode = true
+	if qStats.BIEngineStatistics != nil {
+		expectedMode := false
+		for _, m := range []string{"FULL", "PARTIAL", "DISABLED"} {
+			if qStats.BIEngineStatistics.BIEngineMode == m {
+				expectedMode = true
+			}
+		}
+		if !expectedMode {
+			t.Errorf("unexpected BIEngineMode for BI Engine statistics, got %s", qStats.BIEngineStatistics.BIEngineMode)
 		}
-	}
-	if !expectedMode {
-		t.Errorf("unexpected BIEngineMode for BI Engine statistics, got %s", qStats.BIEngineStatistics.BIEngineMode)
 	}
 }