ImageMagick: Fix best fit resize effect

Only apply `^` operator (minimum values of size) if both width and height are supplied, otherwise the image will be cropped to a square.

src/Charcoal/Image/Imagemagick/Effect/ImagemagickResizeEffect.php

@@ -49,7 +49,14 @@ protected function doResize($width, $height, $bestFit = false)
 
             $size = $width.'x'.$height;
             if ($bestFit) {
-                $params[] = $option.' "'.$size.'^"';
+                if ($width && $height) {
+                    // Minimum values of width and height given, aspect ratio preserved.
+                    $operator = '^';
+                } else {
+                    $operator = '';
+                }
+
+                $params[] = $option.' "'.$size.$operator.'"';
                 $params[] = '-extent '.$size;
             } else {
                 $params[] = $option.' '.$size;