Don't choke on failed node name parsing

tests/test-schema.lua

@@ -44,4 +44,12 @@ describe("schema", function()
 			expect.equal(expected, actual)
 		end)
 	end)
+
+	describe("parse_node_name", function()
+		it("returns NULL on non-sexp table names", function()
+			for val in db:urows("SELECT parse_node_name('not a sexp')") do
+				expect.not_exist(val)
+			end
+		end)
+	end)
 end)