Make emscripten::val default constructible (#17384)

The default ctor of `val` was deleted and it could not be default constructed.
This limited some usages such as create an array of it but without an explicit
 initialization list. (More: https://en.wikipedia.org/wiki/Default_constructor)

The default constructed val is same as val::undefined().

Author: xbcnn

system/include/emscripten/val.h

@@ -399,7 +399,7 @@ class val {
             argv);
     }
 
-    val() = delete;
+    val() : handle(EM_VAL(internal::_EMVAL_UNDEFINED)) {}
 
     explicit val(const char* v)
         : handle(internal::_emval_new_cstring(v))

tests/embind/test_val.cpp

@@ -141,6 +141,22 @@ int main() {
   val::global().set("a", val("1"));
   ensure_js("a == '1'");
 
+  test("val()");
+  val a;
+  val::global().set("a", a);
+  ensure_js("a === undefined");
+  a = val(1);
+  val::global().set("a", a);
+  ensure_js("a == 1");
+  val ar[2];
+  ar[0] = val(1);
+  ar[1] = val(2);
+  val::global().set("a", val::array(ar, ar + 2));
+  ensure_js("a instanceof Array");
+  ensure_js_not("a instanceof Number");
+  ensure_js("a[0] == 1");
+  ensure_js("a[1] == 2");
+
   test("bool isNull()");
   EM_ASM(
     a = null;

tests/embind/test_val.out

@@ -7,6 +7,7 @@ test: val null()...
 test: val global(const char* name = 0)...
 test: template<typename T> explicit val(T&& value)...
 test: val(const char* v)...
+test: val()...
 test: bool isNull()...
 test: bool isUndefined()...
 test: bool isTrue()...