RealTimeGet NPE [LUCENE-7182]

when i use http://solr_ip:8983/solr/collection1/get?id=123456, solr throw nullpoint exception:
null:java.lang.NullPointerException
at org.apache.solr.handler.component.RealTimeGetComponent.createSubRequests(RealTimeGetComponent.java:407)
at org.apache.solr.handler.component.RealTimeGetComponent.distributedProcess(RealTimeGetComponent.java:364)
at org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:322)
at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:155)
at org.apache.solr.core.SolrCore.execute(SolrCore.java:2082)
at org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:670)
at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:458)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:225)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:183)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:215)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:110)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
at org.eclipse.jetty.server.Server.handle(Server.java:499)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:310)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:540)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
at java.lang.Thread.run(Thread.java:745)
but,when when i use http://solr_ip:8983/solr/collection1/get?id=123456&shards=shard1, solr will return the correct result!

---
Migrated from [LUCENE-7182](https://issues.apache.org/jira/browse/LUCENE-7182) by wjw465150
Environment:
```
centos x64 6.3 jdk 1.7
```

Start at org.apache.solr.handler.component.RealTimeGetComponent.createSubRequests in RealTimeGetComponent.java:407 and reproduce the request without shards, then compare it with the working request that specifies shards=shard1. Done means the request without shards no longer throws a NullPointerException and returns the correct result.

Written by the indexing model from the issue text.