fix bug on empty error union

src/ir.cpp

@@ -8128,7 +8128,6 @@ static ZigType *get_error_set_union(CodeGen *g, ErrorTableEntry **errors, ZigTyp
         }
     }
     assert(index == count);
-    assert(count != 0);
 
     if (type_name == nullptr) {
         buf_appendf(&err_set_type->name, "}");

test/stage1/behavior/error.zig

@@ -160,6 +160,10 @@ fn testErrToIntWithOnePossibleValue(
     }
 }
 
+test "empty error union" {
+    const x = error{} || error{};
+}
+
 test "error union peer type resolution" {
     testErrorUnionPeerTypeResolution(1);
 }