feat: narrow return type for min(), max(), median() and mean()

Consider whether the input is empty or not

src/Psl/Math/max.php

@@ -12,7 +12,7 @@
  *
  * @param list<T> $numbers
  *
- * @return T|null
+ * @return ($numbers is non-empty-list<T> ? T : null)
  *
  * @pure
  */

src/Psl/Math/mean.php

@@ -13,6 +13,8 @@
  *
  * @param list<int|float> $numbers
  *
+ * @return ($numbers is non-empty-list<int|float> ? float : null)
+ *
  * @pure
  */
 function mean(array $numbers): ?float

src/Psl/Math/median.php

@@ -14,6 +14,8 @@
  *
  * @param list<int|float> $numbers
  *
+ * @return ($numbers is non-empty-list<int|float> ? float : null)
+ *
  * @pure
  */
 function median(array $numbers): ?float

src/Psl/Math/min.php

@@ -12,7 +12,7 @@
  *
  * @param list<T> $numbers
  *
- * @return T|null
+ * @return ($numbers is non-empty-list<T> ? T : null)
  *
  * @pure
  */