UnifiedHighlighter cannot handle SpanMultiTermQueryWrapper with an Automaton of type SINGLE [LUCENE-9274]

MultiTermHighlighting.extractAutomata ignores a Term from a SINGLE Automaton and Highlighting does not work.

Of course an AutomatonQuery with a single term does not make much sense, but it may be generated by an automatic process.

Possible fixes:
- Either implement consumeTerms in MultiTermHighlighting.AutomataCollector
- Or remove special case for SINGLE in CompiledAutomaton.visit

I attatch a Unit Test

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Migrated from [LUCENE-9274](https://issues.apache.org/jira/browse/LUCENE-9274) by Christoph Goller, updated Mar 12 2020
Attachments: [TestUnifiedHighlighterMTQ.java](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-9274/TestUnifiedHighlighterMTQ.java)

Start by reading MultiTermHighlighting.extractAutomata and its AutomataCollector, then compare the SINGLE handling in CompiledAutomaton.visit. Use the attached TestUnifiedHighlighterMTQ.java as the reproduction and verify that UnifiedHighlighter highlights terms from a SpanMultiTermQueryWrapper using a SINGLE automaton.

Written by the indexing model from the issue text.