fix unique for empty point sets

dbms/src/AggregateFunctions/AggregateFunctionHistogram.h

@@ -159,6 +159,9 @@ private:
      */
     void unique()
     {
+        if (size == 0)
+            return;
+
         size_t left = 0;
 
         for (auto right = left + 1; right < size; right++)

dbms/tests/queries/0_stateless/00647_histogram.reference

@@ -5,3 +5,4 @@
 0.36	0.99	4.75
 -1	1	80
 [(-2,-2,1)]
+[]

dbms/tests/queries/0_stateless/00647_histogram.sql

@@ -6,3 +6,4 @@ WITH arrayJoin(histogram(1)(sin(number-40))) AS res SELECT round(res.1, 2), roun
 
 SELECT histogram(10)(-2);
 
+select histogramIf(3)(number, number > 11) from (select * from system.numbers limit 10);