Prevent "Undefined offset" notice

Fixes an issue where, when an optional argument was at the final position in the argument list, a notice was being generated:

```
$ pho --reporter
PHP Notice:  Undefined offset: 1 in /Users/nstory/.composer/vendor/danielstjules/pho/src/Console/ConsoleOptionParser.php on line 150
PHP Stack trace:
PHP   1. {main}() /Users/nstory/.composer/vendor/danielstjules/pho/bin/pho:0
PHP   2. require_once() /Users/nstory/.composer/vendor/danielstjules/pho/bin/pho:28
PHP   3. pho\Console\Console->parseArguments() /Users/nstory/.composer/vendor/danielstjules/pho/src/pho.php:137
PHP   4. pho\Console\ConsoleOptionParser->parseArguments() /Users/nstory/.composer/vendor/danielstjules/pho/src/Console/Console.php:130

Notice: Undefined offset: 1 in /Users/nstory/.composer/vendor/danielstjules/pho/src/Console/ConsoleOptionParser.php on line 150

Call Stack:
    0.0008     228816   1. {main}() /Users/nstory/.composer/vendor/danielstjules/pho/bin/pho:0
    0.0022     372704   2. require_once('/Users/nstory/.composer/vendor/danielstjules/pho/src/pho.php') /Users/nstory/.composer/vendor/danielstjules/pho/bin/pho:28
    0.0029     473216   3. pho\Console\Console->parseArguments() /Users/nstory/.composer/vendor/danielstjules/pho/src/pho.php:137
    0.0030     490104   4. pho\Console\ConsoleOptionParser->parseArguments() /Users/nstory/.composer/vendor/danielstjules/pho/src/Console/Console.php:130
```