fix: crash when tags value are nullptr

[dmehala]

Description

Fix issue where empty tag values cause an unhandled exception during string construction.

This commit ensures that string building is prevented if a C-string pointer is empty, avoiding potential crashes.

[dubloom]

If you look at the docs :

• using map->merge(source), if a key is already present in map, it won't be extracted from source. Therefore the base code overrides the value from the initial map
• using map->emplace, if the element is already in the map, it is not inserted. Therefore, if the tags map already has one of the key, it will be keep the value it had.

Is this change of behavior desired ?

[dmehala]

@dubloom Yes.