NPE if main query has no match when enable debugQuery for mlt handler

http://localhost:8983/solr/mlt?q=name:aaaa&mlt.fl=name,address&mlt.mindf=1&mlt.mintf=1&fl=*,score&echoParams=ALL&rows=2&debugQuery=true

<response>
<lst name="responseHeader">
<int name="status">0</int>
<int name="QTime">3</int>
<lst name="params">
<str name="mlt.mindf">1</str>
<str name="mlt.fl">name,address</str>
<str name="fl">*,score</str>
<str name="echoParams">ALL</str>
<str name="debugQuery">true</str>
<str name="q">name:aaaa</str>
<str name="mlt.mintf">1</str>
<str name="rows">2</str>
</lst>
</lst>
<result name="match" numFound="0" start="0" maxScore="0.0"/>
<null name="response"/>
<str name="exception_during_debug">
java.lang.RuntimeException: java.lang.NullPointerException at org.apache.solr.search.QueryParsing.toString(QueryParsing.java:575) at org.apache.solr.util.SolrPluginUtils.doStandardQueryDebug(SolrPluginUtils.java:263) at org.apache.solr.util.SolrPluginUtils.doStandardDebug(SolrPluginUtils.java:242) at org.apache.solr.handler.MoreLikeThisHandler.handleRequestBody(MoreLikeThisHandler.java:236) at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:129) at org.apache.solr.core.SolrCore.execute(SolrCore.java:1699) at org.apache.solr.servlet.SolrDispatchFilter.execute(SolrDispatchFilter.java:455) at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:276) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1337) at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:484) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:119) at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:524) at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:233) at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1065) at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:413) at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:192) at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:999) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:117) at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:250) at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:149) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:111) at org.eclipse.jetty.server.Server.handle(Server.java:351) at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:454) at org.eclipse.jetty.server.BlockingHttpConnection.handleRequest(BlockingHttpConnection.java:47) at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:890) at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:944) at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:634) at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:230) at org.eclipse.jetty.server.BlockingHttpConnection.handle(BlockingHttpConnection.java:66) at org.eclipse.jetty.server.bio.SocketConnector$ConnectorEndPoint.run(SocketConnector.java:254) at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:599) at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:534) at java.lang.Thread.run(Thread.java:680) Caused by: java.lang.NullPointerException at org.apache.solr.search.QueryParsing.toString(QueryParsing.java:544) at org.apache.solr.search.QueryParsing.toString(QueryParsing.java:572) ... 32 more
</str>