Context query with regex "." produces an assertion failure [LUCENE-8288]

When a RegexCompletionQuery of "." is provided to ContextQuery, the following assertion failure occurs:

```java
java.lang.AssertionError: input should not end with a context separator followed by SEP_LABEL

at org.apache.lucene.search.suggest.document.ContextQuery$ContextCompletionWeight.setInnerWeight(ContextQuery.java:299)
at org.apache.lucene.search.suggest.document.ContextQuery$ContextCompletionWeight.setNextMatch(ContextQuery.java:275)
at org.apache.lucene.search.suggest.document.NRTSuggester.lookup(NRTSuggester.java:221)
at org.apache.lucene.search.suggest.document.CompletionScorer.score(CompletionScorer.java:70)
at org.apache.lucene.search.BulkScorer.score(BulkScorer.java:39)
at org.apache.lucene.search.suggest.document.SuggestIndexSearcher.suggest(SuggestIndexSearcher.java:78)
at org.apache.lucene.search.suggest.document.SuggestIndexSearcher.suggest(SuggestIndexSearcher.java:58)
at org.apache.lucene.search.suggest.document.TestContextQuery.testDotRegexQuery(TestContextQuery.java:188)
```

Note that this is a related, but distinct issue from #9334, where the RegexCompletionQuery is empty.

The attached patch provides a reproduction of the issue, as the test case TestContextQuery#testRegexDotQuery. To reproduce, Java assertions must be enabled (as in the default configuration for tests).

The patch also provides a test case for the normal behavior of an empty RegexCompletionQuery, when it is not wrapped in ContextQuery (TestRegexCompletionQuery#testRegexDotQuery). In this case, there is no error, and all suggestions are returned.

From a quick look, it seems as though "." doesn't capture any characters past  CompletionAnalyzer.SEP_LABEL, so the matching prefix in ContextCompletionWeight#setInnerWeight is unexpectedly empty.

---
Migrated from [LUCENE-8288](https://issues.apache.org/jira/browse/LUCENE-8288) by Julie Tibshirani (@jtibshirani), updated May 07 2018
Attachments: [LUCENE-8288.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8288/LUCENE-8288.patch) (versions: 2), [LUCENE-8288-repro.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8288/LUCENE-8288-repro.patch)

Start with TestContextQuery#testDotRegexQuery and reproduce the failure with Java assertions enabled. Read ContextQuery$ContextCompletionWeight#setInnerWeight and the related ContextQuery and NRTSuggester stack locations, then compare TestRegexCompletionQuery#testDotRegexQuery. Done means the ContextQuery case no longer asserts while the standalone regex test still returns all suggestions.

Written by the indexing model from the issue text.