fix #14350, cstrings in JS init as null (#14355)

* fix #14350, cstrings in JS init as null

tests/js/tmangle.nim

@@ -62,8 +62,8 @@ block:
     result = result and obj1.`&&`.addr[] == "bar".cstring
     result = result and obj2.`if` == 0
     result = result and obj2.`for` == 0
-    result = result and obj2.`==`.len == 0
-    result = result and obj2.`&&`.len == 0
+    result = result and obj2.`==`.isNil
+    result = result and obj2.`&&`.isNil
   echo test()
 
 # Test codegen for fields with uppercase letters:

tests/js/trepr.nim

@@ -322,7 +322,7 @@ o = [Field0 = [a = "",
 b = @[]],
 Field1 = ""],
 p = nil,
-q = ""]
+q = nil]
 """)
   doAssert(repr(cc) == """
 [a = 12,