Syringa vulgaris

You should have pruned it

I don't think you will do any harm by pruning it now - but not too hard, If it had flowered, this would be the correct time, after all.

Agree both. Unless it's in alkaline soil.

I`ve read that you shouldnt prune in its first year, but can after that, and it will recover from hard pruning.

I believe lilacs flower best in a sunny position. This year ours had flowers only on the unshaded side, it has gradually been overshadowed by nearby conifer hedge and wild cherry trees,. We have cut back the conifers and cherry trees to give it more sunlight and hope this and judicious pruning will help it have more flowers next year.