Cannot copy text consisting only of whitespace

## Abstract
Strings consisting only of whitespace are not copied to the clipboard, and instead the clipboard is cleared.

### Steps to reproduce

```
$ termux-clipboard-set TEST
$ termux-clipboard get
TEST
$ printf '\r\n\r\t\n \r' | termux-clipboard set
$ termux-clipboard-get
```

### Expected output
A string identical to that passed to printf

### Actual output
A single newline