debug: SmartNull dumps as [value] => instead of internal properties

print_r on a SmartNull was the one dump left showing private internals
(useSmartStrings, root, ...). Now it shows one [value] line like its
siblings - the key matches ->value(), same as SmartString's dump.

Author: daveedis

CHANGELOG.md

@@ -7,7 +7,7 @@
 ## [Unreleased]
 
 ### Changed
-- `print_r()` and `var_dump()` now show just the array data, like dumping a plain array. The injected pseudo-properties (the README help pointer and the `useSmartStrings` flag) are gone - the class name PHP prints on every dumped object already identifies the mode, and `var_dump()` rendered the fake entries as broken-looking keys. Use `->debug()` for exact types and metadata. Matches the same change in SmartString 3.0.0.
+- `print_r()` and `var_dump()` now show just the array data, like dumping a plain array. The injected pseudo-properties (the README help pointer and the `useSmartStrings` flag) are gone - the class name PHP prints on every dumped object already identifies the mode, and `var_dump()` rendered the fake entries as broken-looking keys. Use `->debug()` for exact types and metadata. `SmartNull` dumps as `[value] =>` instead of exposing its internal properties. Matches the same change in SmartString 3.0.0.
 - Deprecated method names (`toRaw()`, `toHtml()`, `withSmartStrings()`, `enableSmartStrings()`, `noSmartStrings()`, `disableSmartStrings()`, `isMultipleOf()`, plus `smartMap()` and `chunk()`) are now real declared methods marked `@deprecated`, organized by deprecation stage in one `DeprecatedAliases` trait, instead of `__call()` shims. IDEs now show strikethroughs with the replacement, `method_exists()` reports them, and calls skip `__call()` dispatch. Same behavior and deprecation notices as before.
 - `set()`, `->key = $value`, and array assignment now unwrap Smart values (SmartString, SmartArray, SmartNull) instead of throwing, so values can be copied between arrays in any mode without calling `->value()` first. SmartNull stores as null; nested SmartArrays convert to the target array's mode. Matches how `where()`, `contains()`, and `merge()` already treat Smart inputs.
 - `SmartArray::new($data, true)` and `SmartArrayHtml::new($data, false)` now throw like the constructors do, instead of silently ignoring the boolean. Old code that passed `true` expecting auto-encoding was silently getting raw, unencoded values - now it fails at the call site with the class to use instead. Redundant booleans (`false` on SmartArray, `true` on SmartArrayHtml) log a deprecation and proceed.

src/SmartNull.php

@@ -110,6 +110,15 @@ public function help(): void
         echo $output;
     }
 
+    /**
+     * Shows just the null value in print_r() and var_dump() output, hiding internal
+     * properties. Matches SmartString's [value] dump.
+     */
+    public function __debugInfo(): array
+    {
+        return ['value' => null];
+    }
+
     //endregion
     //region Iterator Methods