CBL-4348 : Tag prop encryptor and decryptor as nullable (#485)

Tagged propertyEncryptor, propertyDecryptor, documentPropertyEncryptor, and documentPropertyDecryptor as nullable. They are optional.

Ported from baad2f1 in branch release/3.1.

include/cbl/CBLReplicator.h

@@ -411,15 +411,18 @@ typedef struct {
     /** Optional callback to encrypt \ref CBLEncryptable values of the documents in the default collection.
         @note This property can only be used when setting the config object with the database instead of collections.
         @warning  <b>Deprecated :</b> Use documentPropertyEncryptor instead. */
-    CBLPropertyEncryptor propertyEncryptor;
+    CBLPropertyEncryptor _cbl_nullable propertyEncryptor;
 
     /** Optional callback to decrypt encrypted \ref CBLEncryptable values of the documents in the default collection.
         @note This property can only be used when setting the config object with the database instead of collections.
         @warning  <b>Deprecated :</b> Use documentPropertyDecryptor instead. */
-    CBLPropertyDecryptor propertyDecryptor;
+    CBLPropertyDecryptor _cbl_nullable propertyDecryptor;
 
-    CBLDocumentPropertyEncryptor documentPropertyEncryptor;     ///< Optional callback to encrypt \ref CBLEncryptable values.
-    CBLDocumentPropertyDecryptor documentPropertyDecryptor;     ///< Optional callback to decrypt encrypted \ref CBLEncryptable values.
+    /** Optional callback to encrypt \ref CBLEncryptable values. */
+    CBLDocumentPropertyEncryptor _cbl_nullable documentPropertyEncryptor;
+
+    /** Optional callback to decrypt encrypted \ref CBLEncryptable values. */
+    CBLDocumentPropertyDecryptor _cbl_nullable documentPropertyDecryptor;
 #endif
 
     /** The collections to replicate with the target's endpoint (Required if the database is not set). */