[BUGFIX] Mitigate "Using null as an array offset is deprecated"

Using `DocBlockFactory->create()` with a phpdocblock
hanging a `@throws` tag followed by a exception class
name emits following PHP 8.5.0 deprecation:

```
Using null as an array offset is deprecated,
use an empty string instead
```

in `StandardTagFactory->getArgumentsForParametersFromWiring()`.

This change uses the null-coalsce operator to fallback to
an empty string for the value which should be used as an
array key instead of using `null` to mitigate the deprecation
notice with PHP8.5.0 and matches the behaviour in earlier
PHP versions without the deprecation message.

tests/unit/DocBlock/StandardTagFactoryTest.php

@@ -525,6 +525,11 @@ class StandardTagFactoryTest extends TestCase
                 'tag',
                 '@tag (is valid)',
             ],
+            'full-qualified-class-name following a tag name is valid' => [
+                '@tag \InvalidArgumentException',
+                'tag',
+                '@tag \InvalidArgumentException',
+            ],
         ];
     }