Initialize newString in js [backport:1.6] (nim-lang#22745)

```nim
echo newString(8)
```

results in:
```
D:\User\test.js:25
                  var code_33556944 = c_33556931.toString(16);
                                                 ^

TypeError: Cannot read properties of undefined (reading 'toString')
    at toJSStr (D:\User\test.js:25:50)
    at rawEcho (D:\User\test.js:70:16)
    at Object.<anonymous> (D:\User\test.js:101:1)
    at Module._compile (node:internal/modules/cjs/loader:1095:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1147:10)
    at Module.load (node:internal/modules/cjs/loader:975:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    at node:internal/main/run_main_module:17:47

Node.js v17.0.1
Error: execution of an external program failed: '"C:\Program Files\nodejs\node.exe" --unhandled-rejections=strict D:\User\test.js'
```

Author: AmjadHD

lib/system/jssys.nim

@@ -278,7 +278,9 @@ proc toJSStr(s: string): cstring {.compilerproc.} =
 
 proc mnewString(len: int): string {.asmNoStackFrame, compilerproc.} =
   asm """
-    return new Array(`len`);
+    var result = new Array(`len`);
+    for (var i = 0; i < `len`; i++) {result[i] = 0;}
+    return result;
   """
 
 proc SetCard(a: int): int {.compilerproc, asmNoStackFrame.} =