When Graph Item is not linked to a data source, Comments do not always get variables substituted correctly

[ddb4github]

Describe the bug

After #2656, rrd_substitute_host_query_data function substitute graph item text_format from data_local.snmp_query_id only.
If add a COMMENT item without any associated datasource, graph will show as screenshot below.

To Reproduce

Steps to reproduce the behavior:

1. Edit build-in graph template "Unix - Available Disk Space"
2. Add a item as below:
   
3. Create a graph from DataQuery "Unix - Get Mounted Partitions " under build-in device "Local Linux Machine"
4. View Graph after while
5. See error as screenshot below

Expected behavior

|query_xxxx| always be replaced for related data query, whatever from graph or data source.

Screenshots