Escape null characters as \x00

Since \0 might be mistakenly interpreted as octal if followed by a number and using literal null is in some cases interpreted as end of string, escape null as \x00.

For example, the following JS

```javascript
"\x001"
```

is before this patch minified as 

```javascript
"\01"
```

Depending on whether strict mode is enabled or not, this either results in syntax error or equivalent of `\x01`. PR fixes this edge case behaviour and should [fix minification of pdf.js](https://github.com/mozilla/pdf.js/issues/2479).