Fix bug causing all documentation to be omitted

[jnkr-ifx]

The matching filter searches for a partial regex match, and the empty regex matches anything.

Description

f015ea4 added the following check to skip generating a #[doc = "..."] attribute for items with no documentation:

{%- if doc_string is not matching("") %} 
#[doc = "{{doc_string}}"]
{%- endif -%}

However, the matching filter uses a regex match, and the empty regex matches everything. Therefore, all documentation was skipped. This PR replaces the regex with a string comparison against the empty string.

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