Context value declaration, no query

BaseX stops executing when the following incomplete query is run: ```sh basex "declare context value := doc('x');" ``` ``` Exception in thread "main" java.lang.NullPointerException: Cannot invoke "org.basex.query.expr.Expr.accept(org.basex.query.util.ASTVisitor)" because "this.expr" is null at org.basex.query.expr.Single.accept(Single.java:74) at org.basex.query.scope.MainModule.databases(MainModule.java:93) at org.basex.query.QueryContext.addLocks(QueryContext.java:427) at org.basex.query.QueryProcessor.addLocks(QueryProcessor.java:231) at org.basex.core.cmd.AQuery.addLocks(AQuery.java:172) at org.basex.core.locks.Locking.acquire(Locking.java:86) at org.basex.core.jobs.Job.register(Job.java:48) at org.basex.core.Command.execute(Command.java:90) at org.basex.api.client.LocalSession.execute(LocalSession.java:131) at org.basex.api.client.Session.execute(Session.java:36) at org.basex.core.CLI.execute(CLI.java:94) at org.basex.BaseX.<init>(BaseX.java:111) at org.basex.BaseX.main(BaseX.java:44) ```