Hexadecimal is never an empty string (#593)

Similarly to UUIDs, a hexadecimal can never be empty, this PR documents
it as such.

More and more libraries are starting to enforce stricter typing, I was
dealing with a library that required a `non-empty-string` which
hexadecimal isn't (is, but isn't documented that it is) which requires
either baselining false positives are adding redundant assertions.

src/Type/Hexadecimal.php

@@ -32,6 +32,9 @@ use function substr;
  */
 final class Hexadecimal implements TypeInterface
 {
+    /**
+     * @var non-empty-string
+     */
     private string $value;
 
     /**
@@ -42,21 +45,33 @@ final class Hexadecimal implements TypeInterface
         $this->value = $value instanceof self ? (string) $value : $this->prepareValue($value);
     }
 
+    /**
+     * @psalm-return non-empty-string
+     */
     public function toString(): string
     {
         return $this->value;
     }
 
+    /**
+     * @psalm-return non-empty-string
+     */
     public function __toString(): string
     {
         return $this->toString();
     }
 
+    /**
+     * @psalm-return non-empty-string
+     */
     public function jsonSerialize(): string
     {
         return $this->toString();
     }
 
+    /**
+     * @psalm-return non-empty-string
+     */
     public function serialize(): string
     {
         return $this->toString();
@@ -94,6 +109,9 @@ final class Hexadecimal implements TypeInterface
         $this->unserialize($data['string']);
     }
 
+    /**
+     * @psalm-return non-empty-string
+     */
     private function prepareValue(string $value): string
     {
         $value = strtolower($value);
@@ -108,6 +126,7 @@ final class Hexadecimal implements TypeInterface
             );
         }
 
+        /** @var non-empty-string */
         return $value;
     }
 }