Nette\ImageMagick constructor fix

This patch fixes a PHP warning introduced by [this pull request](https://github.com/nette/nette/commit/1bae714ada601fb02538df10c136ba89b7db5b15) in [nette/nette](https://github.com/nette/nette). Basically contructor doesn't check for first argument expecting it to be a string, but this class can be instantiated from a parent static method e. g. `::fromBlank()`. Such call results in warning.