Fix a crash when calculating an offset for nil string.

Calling -containsString: with nil argument throws an exception, so this
check is needed.

Bug: 880590
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ieb3944b05fdf3573f6a552c8910025bf93312987
Reviewed-on: https://chromium-review.googlesource.com/c/1225983Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596650}

ios/chrome/browser/ui/omnibox/omnibox_text_field_ios.mm

@@ -365,7 +365,7 @@ NSString* const kOmniboxFadeAnimationKey = @"OmniboxFadeAnimation";
   // default to a reasonable prefix string to give a plausible offset.
   NSString* prefixString = @"https://";
 
-  if ([self.text containsString:string]) {
+  if (string.length > 0 && [self.text containsString:string]) {
     NSRange range = [self.text rangeOfString:string];
     NSRange prefixRange = NSMakeRange(0, range.location);
     prefixString = [self.text substringWithRange:prefixRange];