Fix some project warnings + other fixes

- Prefer checking `.isEmpty` rather than `.count == 0` for Swift collections (can be more optimal)
- Avoid comparing `BOOL` explicitly to `NO` (considered bad practice since `BOOL` is a `signed char`)
- Specify unused `SGX_FLAGS` as `__unused` to silence warnings

Signal/src/ViewControllers/ContactsPicker.swift

@@ -328,7 +328,7 @@ public class ContactsPicker: OWSViewController, UITableViewDelegate, UITableView
 
     open func updateSearchResults(searchText: String) {
         let predicate: NSPredicate
-        if searchText.count == 0 {
+        if searchText.isEmpty {
             filteredSections = sections
         } else {
             do {

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -4013,7 +4013,7 @@ typedef enum : NSUInteger {
 
 - (void)saveDraft
 {
-    if (self.inputToolbar.hidden == NO) {
+    if (!self.inputToolbar.hidden) {
         __block TSThread *thread = _thread;
         __block NSString *currentDraft = [self.inputToolbar messageText];
 

Signal/src/ViewControllers/MessageDetailViewController.swift

@@ -362,7 +362,7 @@ class MessageDetailViewController: OWSViewController, MediaGalleryDataSourceDele
         self.messageBubbleViewHeightLayoutConstraint = messageBubbleView.autoSetDimension(.height, toSize: 0)
         rows.append(row)
 
-        if rows.count == 0 {
+        if rows.isEmpty {
             // Neither attachment nor body.
             owsFail("\(self.logTag) Message has neither attachment nor body.")
             rows.append(valueRow(name: NSLocalizedString("MESSAGE_METADATA_VIEW_NO_ATTACHMENT_OR_BODY",

SignalMessaging/environment/OWSAudioSession.swift

@@ -141,7 +141,7 @@ public class OWSAudioSession: NSObject {
             return oldActivity
         }
 
-        guard currentActivities.count == 0 else {
+        guard currentActivities.isEmpty else {
             Logger.debug("\(logTag) not deactivating due to currentActivities: \(currentActivities)")
             return
         }

SignalServiceKit/src/Contacts/CDSQuote.m

@@ -10,11 +10,11 @@ NS_ASSUME_NONNULL_BEGIN
 static const long SGX_FLAGS_INITTED = 0x0000000000000001L;
 static const long SGX_FLAGS_DEBUG = 0x0000000000000002L;
 static const long SGX_FLAGS_MODE64BIT = 0x0000000000000004L;
-static const long SGX_FLAGS_PROVISION_KEY = 0x0000000000000004L;
-static const long SGX_FLAGS_EINITTOKEN_KEY = 0x0000000000000004L;
+static const long __unused SGX_FLAGS_PROVISION_KEY = 0x0000000000000004L;
+static const long __unused SGX_FLAGS_EINITTOKEN_KEY = 0x0000000000000004L;
 static const long SGX_FLAGS_RESERVED = 0xFFFFFFFFFFFFFFC8L;
-static const long SGX_XFRM_LEGACY = 0x0000000000000003L;
-static const long SGX_XFRM_AVX = 0x0000000000000006L;
+static const long __unused SGX_XFRM_LEGACY = 0x0000000000000003L;
+static const long __unused SGX_XFRM_AVX = 0x0000000000000006L;
 static const long SGX_XFRM_RESERVED = 0xFFFFFFFFFFFFFFF8L;
 
 #pragma mark -