Do not create histograms for single column unique key

The intentention was always to not create histograms for single value
unique keys (as histograms is not useful in this case), but because of
a bug in the code this was still done.

The changes in the test cases was mainly because hist_size is now NULL
for these kind of columns.

mysql-test/main/stat_tables.result

@@ -542,14 +542,14 @@ test.t1	analyze	Warning	Engine-independent statistics are not collected for colu
 test.t1	analyze	status	OK
 SELECT * FROM mysql.column_stats;
 db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
-test	t1	pk	1	2	0.0000	4.0000	1.0000	0	NULL	NULL
+test	t1	pk	1	2	0.0000	4.0000	1.0000	NULL	NULL	NULL
 DELETE FROM mysql.column_stats
 WHERE db_name='test' AND table_name='t1' AND column_name='t';
 INSERT INTO mysql.column_stats VALUES
 ('test','t1','t','bar','foo', 0.0, 3.0, 1.0, 0, NULL, NULL);
 SELECT * FROM mysql.column_stats;
 db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
-test	t1	pk	1	2	0.0000	4.0000	1.0000	0	NULL	NULL
+test	t1	pk	1	2	0.0000	4.0000	1.0000	NULL	NULL	NULL
 test	t1	t	bar	foo	0.0000	3.0000	1.0000	0	NULL	NULL
 SELECT pk FROM t1;
 pk
@@ -573,7 +573,7 @@ pk	c
 2	bar
 SELECT * FROM mysql.column_stats;
 db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
-test	t1	pk	1	2	0.0000	4.0000	1.0000	0	NULL	NULL
+test	t1	pk	1	2	0.0000	4.0000	1.0000	NULL	NULL	NULL
 test	t1	c	bar	foo	0.0000	3.0000	1.0000	0	NULL	NULL
 CREATE OR REPLACE TABLE t1 (pk int PRIMARY KEY, a char(7));
 SELECT * FROM t1;

mysql-test/main/stat_tables_innodb.result

@@ -574,14 +574,14 @@ test.t1	analyze	Warning	Engine-independent statistics are not collected for colu
 test.t1	analyze	status	OK
 SELECT * FROM mysql.column_stats;
 db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
-test	t1	pk	1	2	0.0000	4.0000	1.0000	0	NULL	NULL
+test	t1	pk	1	2	0.0000	4.0000	1.0000	NULL	NULL	NULL
 DELETE FROM mysql.column_stats
 WHERE db_name='test' AND table_name='t1' AND column_name='t';
 INSERT INTO mysql.column_stats VALUES
 ('test','t1','t','bar','foo', 0.0, 3.0, 1.0, 0, NULL, NULL);
 SELECT * FROM mysql.column_stats;
 db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
-test	t1	pk	1	2	0.0000	4.0000	1.0000	0	NULL	NULL
+test	t1	pk	1	2	0.0000	4.0000	1.0000	NULL	NULL	NULL
 test	t1	t	bar	foo	0.0000	3.0000	1.0000	0	NULL	NULL
 SELECT pk FROM t1;
 pk
@@ -605,7 +605,7 @@ pk	c
 2	bar
 SELECT * FROM mysql.column_stats;
 db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
-test	t1	pk	1	2	0.0000	4.0000	1.0000	0	NULL	NULL
+test	t1	pk	1	2	0.0000	4.0000	1.0000	NULL	NULL	NULL
 test	t1	c	bar	foo	0.0000	3.0000	1.0000	0	NULL	NULL
 CREATE OR REPLACE TABLE t1 (pk int PRIMARY KEY, a char(7));
 SELECT * FROM t1;

sql/sql_statistics.cc

@@ -2345,7 +2345,7 @@ void Column_statistics_collected::init(THD *thd, Field *table_field)
   column_total_length= 0;
   if (is_single_pk_col)
     count_distinct= NULL;
-  if (table_field->flags & BLOB_FLAG)
+  else if (table_field->flags & BLOB_FLAG)
     count_distinct= NULL;
   else
   {