Respond to CR.

// FREEBIE

src/Contacts/TSThread.h

@@ -147,7 +147,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)setDraft:(NSString *)draftString transaction:(YapDatabaseReadWriteTransaction *)transaction;
 
 @property (atomic, readonly) BOOL isMuted;
-@property (atomic, readonly) NSDate *mutedUntilDate;
+@property (atomic, readonly, nullable) NSDate *mutedUntilDate;
 
 // This model may be updated from many threads. We don't want to save
 // our local copy (this instance) since it may be out of date.  Instead, we

src/Contacts/TSThread.m

@@ -21,7 +21,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, copy) NSDate *archivalDate;
 @property (nonatomic) NSDate *lastMessageDate;
 @property (nonatomic, copy) NSString *messageDraft;
-@property (atomic) NSDate *mutedUntilDate;
+@property (atomic, nullable) NSDate *mutedUntilDate;
 
 - (TSInteraction *)lastInteraction;