Create query context for clickhouse dictionary source.

dbms/src/Dictionaries/ClickHouseDictionarySource.cpp

@@ -76,6 +76,9 @@ ClickHouseDictionarySource::ClickHouseDictionarySource(
     context.setUser(user, password, Poco::Net::SocketAddress("127.0.0.1", 0), {});
     /// Processors are not supported here yet.
     context.getSettingsRef().experimental_use_processors = false;
+    /// Query context is needed because some code in executeQuery function may assume it exists.
+    /// Current example is Context::getSampleBlockCache from InterpreterSelectWithUnionQuery::getSampleBlock.
+    context.makeQueryContext();
 }
 
 
@@ -100,6 +103,7 @@ ClickHouseDictionarySource::ClickHouseDictionarySource(const ClickHouseDictionar
     , pool{is_local ? nullptr : createPool(host, port, secure, db, user, password)}
     , load_all_query{other.load_all_query}
 {
+    context.makeQueryContext();
 }
 
 std::string ClickHouseDictionarySource::getUpdateFieldAndDate()