`GREYElementMatcherBlock` not respecting that `item` can be nil.

Nobody has claimed this yet.

[GREYBaseMatcher](https://github.com/google/EarlGrey/blob/1d507003e3c436c2db89d0945e3d23f7bf25ac2b/EarlGrey/Matcher/GREYBaseMatcher.h) indicates that the element passed to `- (BOOL)matches:(_Nullable id)item;` can be null. However [GREYElementMatcherBlock](https://github.com/google/EarlGrey/blob/1d507003e3c436c2db89d0945e3d23f7bf25ac2b/EarlGrey/Matcher/GREYElementMatcherBlock.m) defines `MatchesBlock` with a non-null item and does not check the item for nil before calling the block. This results in `EXEC_BAC_ACCESS` when using Swift on Xcode 9.3.

`GREYElementMatcherBlock.m` should change its `matches` method to:

```Objective-C
- (BOOL)matches:(id)item {
if (!item) return NO;
return _matcherBlock(item);
}
```

I'm sorry that I didn't create a PR. I discovered this at work and I have to go through an open source review process to submit a PR. It looks like 1.14.0 is close to being released and getting through my works approval process takes a long time.

Open EarlGrey/Matcher/GREYElementMatcherBlock.m and inspect the matches: method and its MatchesBlock definition. Confirm that a nil item is handled without invoking the block and returns NO, while non-nil matching still behaves as before.

Written by the indexing model from the issue text.