Update Password Samples for Nullability

Issue gh-16226

Author: jzheaux

docs/src/test/kotlin/org/springframework/security/kt/docs/features/authentication/authenticationpasswordstorageargon2/Argon2PasswordEncoderUsage.kt

@@ -8,7 +8,7 @@ class Argon2PasswordEncoderUsage {
         // tag::argon2PasswordEncoder[]
         // Create an encoder with all the defaults
         val encoder = Argon2PasswordEncoder.defaultsForSpringSecurity_v5_8()
-        val result: String = encoder.encode("myPassword")
+        val result: String? = encoder.encode("myPassword")
         assertTrue(encoder.matches("myPassword", result))
         // end::argon2PasswordEncoder[]
     }

docs/src/test/kotlin/org/springframework/security/kt/docs/features/authentication/authenticationpasswordstoragebcrypt/BCryptPasswordEncoderUsage.kt

@@ -8,7 +8,7 @@ class BCryptPasswordEncoderUsage {
         // tag::bcryptPasswordEncoder[]
         // Create an encoder with strength 16
         val encoder = BCryptPasswordEncoder(16)
-        val result: String = encoder.encode("myPassword")
+        val result: String? = encoder.encode("myPassword")
         assertTrue(encoder.matches("myPassword", result))
         // end::bcryptPasswordEncoder[]
     }

docs/src/test/kotlin/org/springframework/security/kt/docs/features/authentication/authenticationpasswordstoragepbkdf2/Pbkdf2PasswordEncoderUsage.kt

@@ -8,7 +8,7 @@ class Pbkdf2PasswordEncoderUsage {
         // tag::pbkdf2PasswordEncoder[]
         // Create an encoder with all the defaults
         val encoder = Pbkdf2PasswordEncoder.defaultsForSpringSecurity_v5_8()
-        val result: String = encoder.encode("myPassword")
+        val result: String? = encoder.encode("myPassword")
         assertTrue(encoder.matches("myPassword", result))
         // end::pbkdf2PasswordEncoder[]
     }

docs/src/test/kotlin/org/springframework/security/kt/docs/features/authentication/authenticationpasswordstoragescrypt/SCryptPasswordEncoderUsage.kt

@@ -8,7 +8,7 @@ class SCryptPasswordEncoderUsage {
         // tag::sCryptPasswordEncoder[]
         // Create an encoder with all the defaults
         val encoder = SCryptPasswordEncoder.defaultsForSpringSecurity_v5_8()
-        val result: String = encoder.encode("myPassword")
+        val result: String? = encoder.encode("myPassword")
         assertTrue(encoder.matches("myPassword", result))
         // end::sCryptPasswordEncoder[]
     }