sql: Add stat spaces during schema_init

If Tarantool is being starteed using data from older
version, where stat tables were not system, it will definitely
crash, since SQL statistics analyzer will try to access non-
existing statistics tables (which will be added during upgrade).

To avoid such crash: create dummy statistics spaces during
schema init.

Also, fix types for statistics space format: prohibit null values
for statistic values and make sample format stronger.