IOS-2628 Deal with "no attachment"

pEpObjCAdapter/Extensions/NSArray+Engine.m

@@ -123,7 +123,16 @@
 
 - (bloblist_t * _Nullable)toBloblist
 {
+    if (self.count == 0) {
+        return NULL;
+    }
+
     bloblist_t *_bl = new_bloblist(NULL, 0, NULL, NULL);
+
+    if (!_bl) {
+        return NULL;
+    }
+
     bloblist_t *bl =_bl;
 
     // free() might be the default, but let's be explicit

pEpObjCAdapter/Extensions/PEPMessage+Engine.m

@@ -91,7 +91,7 @@
                                               precomposedStringWithCanonicalMapping]
                                              UTF8String], 0);
 
-    if (self.attachments && self.attachments.count > 0) {
+    if (self.attachments) {
         msg->attachments = [self.attachments toBloblist];
     }