CheckIndex.checkIndex throws IndexNotFoundException: no segments* file found in RAMDirectory@ [LUCENE-6765]

I am testing upgrade from Lucene 4.1 to 5.2.1 and found that behavior of CheckIndex.checkIndex() changed. We used the following to create status for empty dir (we use it for some reporting purpose):

CheckIndex.Status status = new CheckIndex(new RAMDirectory()).checkIndex();

At 5.2.1 it throws:

org.apache.lucene.index.IndexNotFoundException: no segments\* file found in RAMDirectory@51f090da lockFactory=org.apache.lucene.store.SingleInstanceLockFactory@8b6023d: files: []
at org.apache.lucene.index.CheckIndex.checkIndex(CheckIndex.java:487)
at org.apache.lucene.index.CheckIndex.checkIndex(CheckIndex.java:466)

I expect that it should returns status with cantOpenSegments flag on. https://lucene.apache.org/core/5_2_1/core/org/apache/lucene/index/CheckIndex.Status.html#cantOpenSegments

Is it expected behavior?

If this isn't expected, I am willing to provide a fix (should be minor) that I could figure out on it how to contribute.

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Migrated from [LUCENE-6765](https://issues.apache.org/jira/browse/LUCENE-6765) by Vladimir Kuzmin
Environment:
```
OS X, Lucene 5.2.1
```

Start with CheckIndex.java at the reported checkIndex stack-trace lines and compare the empty RAMDirectory behavior with the Status documentation for cantOpenSegments. Reproduce the example on Lucene 5.2.1, then determine whether an empty directory should return a status or throw IndexNotFoundException; done means the intended behavior is confirmed and covered by a regression test.

Written by the indexing model from the issue text.