fix: do not read an undefined offset when a value argument is last

The argument parser looked up the next command-line token via $argv[$key + 1]
guarded only by count($argv) === 0. The remaining arguments array is not
reindexed after earlier arguments are removed, so a value-taking argument placed
last (after an unconsumed token) read an undefined offset and raised an Undefined
array key warning under PHP 8, and the argument was set to empty instead of
falling back to its default. Check that the next offset exists with isset().

src/Argument/Parser.php

@@ -261,7 +261,9 @@ class Parser
         }
 
         if (is_null($value)) {
-            if (count($argv) === 0) {
+            // The array is not reindexed as arguments are removed, so check the next
+            // offset itself rather than the overall count.
+            if (!isset($argv[$key + 1])) {
                 return $argv;
             }