fix head 0, tail 0, and limit 0 so they work properly

compiler/rungen/op.go

@@ -322,11 +322,7 @@ func (b *Builder) compileLeaf(o dag.Op, parent sbuf.Puller) (sbuf.Puller, error)
 	case *dag.MergeOp:
 		return nil, errors.New("merge: multiple upstream paths required")
 	case *dag.HeadOp:
-		limit := v.Count
-		if limit == 0 {
-			limit = 1
-		}
-		return head.New(parent, limit), nil
+		return head.New(parent, v.Count), nil
 	case *dag.LoadOp:
 		return load.New(b.rctx, b.env.DB(), parent, v.Pool, v.Branch, v.Author, v.Message, v.Meta), nil
 	case *dag.OutputOp:
@@ -364,11 +360,7 @@ func (b *Builder) compileLeaf(o dag.Op, parent sbuf.Puller) (sbuf.Puller, error)
 		}
 		return sort.New(b.rctx, parent, sortExprs, v.Reverse, b.resetters), nil
 	case *dag.TailOp:
-		limit := v.Count
-		if limit == 0 {
-			limit = 1
-		}
-		return tail.New(parent, limit), nil
+		return tail.New(parent, v.Count), nil
 	case *dag.TopOp:
 		b.resetResetters()
 		var sortExprs []expr.SortExpr

compiler/ztests/sql/limit-zero.yaml

@@ -0,0 +1,3 @@
+spq: select 1 limit 0
+
+output: ""

runtime/ztests/op/head-zero.yaml

@@ -0,0 +1,3 @@
+spq: values 1 | head 0
+
+output: ""

runtime/ztests/op/tail-zero.yaml

@@ -0,0 +1,3 @@
+spq: values 1 | tail 0
+
+output: ""