hmget breaks on null values if used with detect_buffers

Hi!

The following code snippet reproduces the problem:

    var redis = require('redis');
    
    var clt = redis.createClient(6379, 'localhost', { detect_buffers : true });
    
    clt.del('foo_129000', function(err) {
        if (err) throw err;
        clt.hmget('foo_129000', 'bar', 'baz', function(err, arr) {
            if (err) throw err;
            console.log(arr);
        });
    });


The pull request patches `reply_to_strings()` to avoid calling `toString()` on null/undefined values

#### Ready to merge checklist
- [x] test(s) in test.js
- [x] tests will fail without the PR, but succeed once applied
- [x] docs, if applicable
- [x] merges cleanly
- [x] coding style (4-space indents, looks similar to other code)