matcherForEnabledElement is treating nil elements as enabled

When using grey_enabled in a test I noticed that any text I used did not assert

```
EarlGrey.select(elementWithMatcher:
allOf(withText("bar"),
withAncestor(allOf(withClass(MyClass.self), isVisible())))).assert(grey_enabled())
```

This is a case where "bar" did not exist at all and I was expecting an assertion thrown.

The answer is here:
https://github.com/google/EarlGrey/blob/b0ed5e03fb2b5a5ba00843811c9e87baa547bf6b/EarlGrey/Matcher/GREYMatchers.m#L476

The default value of BOOL matched should be NO instead of YES.

Inspect EarlGrey/Matcher/GREYMatchers.m around line 476, as linked in the issue, and trace how matcherForEnabledElement handles an element that is not found. Verify the behavior with the grey_enabled example: a missing element should cause the assertion to fail rather than pass.

Written by the indexing model from the issue text.