Number(value) function should return NaN if value is invalid, but returns null.

[ykaragol]

1. Which version of PhantomJS are you using?
   PhantomJS 2.1.1
2. What steps will reproduce the problem?
   Number(value) function should return NaN if value cannot be converted into a number.
   But it returns null.
3. Which operating system are you using?
   Windows 7
4. Did you use binary PhantomJS or did you compile it from source?
   Binary
5. Please provide any additional information below.
   You can test with Number('asd');

[stale]

Due to our very limited maintenance capacity, we need to prioritize our development focus on other tasks. Therefore, this issue will be automatically closed (see #15395 for more details). In the future, if we see the need to attend to this issue again, then it will be reopened. Thank you for your contribution!