Zabbix

.......PS. [ZBX-7208] fixed DB schema generation for "t_shorttext"-type fields

Before this change:
   - If a field type was "t_shorttext" in "create/src/schema.tmpl" then
     the script "create/bin/gen_schema.pl" generated length 0 for this field
     in "src/libs/zbxdbhigh/dbschema.c" array "tables".
     As a consequence Zabbix server refused to write into such zero-length field,
     for example, into "host_inventory" table field "notes".

After this change:
   - The script "create/bin/gen_schema.pl" generates length 2048 for IBM DB2 and Oracle
     databases and length 65535 for other databases for "t_shorttext"-type fields.
   - fixed compiler warning "missing initializer for field 'uniq'".
   - macro ZBX_NULL2STR() added.