Address edge cases for number_to_human with units option.

`ActiveSupport::NumberHelper#number_to_human` returns the number unaltered when the units option is an empty hash or when its smallest key is larger than the number.

Examples:

```
number_to_human(123, :units => {}) # => 123
number_to_human(123, :units => {:thousand => 'k'}) # => 123
```

Fixes #9269.