fix: let lax mode perform emptytable key-value inference as normal

Do not over-constrain types with `unknown`, let `assert_is_a`
perform the check when t2 is `unresolved_emptytable_value`.

spec/lax/lax_spec.lua

@@ -43,4 +43,17 @@ describe("lax mode", function()
       { msg = "x" },
       { msg = "y" },
    }))
+
+   it("performs emptytable key-value inference as normal", util.lax_check([[
+      local t = {}
+
+      local s = "str"
+
+      t[s] = 9
+
+      for k, v in pairs(t) do
+         print(k, v)
+      end
+   ]], {}))
+
 end)

tl.lua

@@ -8503,7 +8503,7 @@ a.types[i], b.types[i]), }
    function TypeChecker:assert_is_a(w, t1, t2, ctx, name)
       t1 = resolve_tuple(t1)
       t2 = resolve_tuple(t2)
-      if self.feat_lax and (is_unknown(t1) or is_unknown(t2)) then
+      if self.feat_lax and (is_unknown(t1) or t2.typename == "unknown") then
          return true
       end
 

tl.tl

@@ -8503,7 +8503,7 @@ do
    function TypeChecker:assert_is_a(w: Where, t1: Type, t2: Type, ctx?: string | Node, name?: string): boolean
       t1 = resolve_tuple(t1)
       t2 = resolve_tuple(t2)
-      if self.feat_lax and (is_unknown(t1) or is_unknown(t2)) then
+      if self.feat_lax and (is_unknown(t1) or t2 is UnknownType) then
          return true
       end