fix: can assign an emptytable to an emptytable

This should also avoid inferring types as `{} | {}`.

spec/declaration/union_spec.lua

@@ -139,4 +139,12 @@ describe("union declaration", function()
       { msg = "cannot discriminate a union between multiple function types" },
    }))
 
+   it("collapses multiple emptytables on declaration", util.check([[
+      local function count_sea_monsters(image: {string})
+         local c, m, n = 0, {{}, {}}, 0
+         for row = 1, #image - 2 do
+            m[row + 2] = {}
+         end
+      end
+   ]]))
 end)

spec/inference/emptytable_spec.lua

@@ -196,4 +196,17 @@ describe("empty table without type annotation", function()
        end
    ]]))
 
+   it("can assign an emptytable to an emptytable", util.check([[
+      local x = {}
+
+      for i = 1, 20 do
+         if math.random(2) == 1 then
+            x = {}
+         else
+            x = {"hello"}
+         end
+         print(#x)
+      end
+   ]]))
+
 end)

tl.lua

@@ -8476,6 +8476,7 @@ do
 
 
    local emptytable_relations = {
+      ["emptytable"] = compare_true,
       ["array"] = compare_true,
       ["map"] = compare_true,
       ["tupletable"] = compare_true,

tl.tl

@@ -8476,6 +8476,7 @@ do
 
    -- emptytable rules are the same in eqtype_relations and subtype_relations
    local emptytable_relations: {TypeName:CompareTypes} = {
+      ["emptytable"] = compare_true,
       ["array"] = compare_true,
       ["map"] = compare_true,
       ["tupletable"] = compare_true,