fix: adjust parsing of app.allowed_env_substitutions

[beiertu-mms]

SmallRyeConfig getValue doesn't allow the provided configured value to
be empty, therefore use getOptionalValue instead.

See also: eclipse/microprofile-config#407

Fixes #303

[technolinator-sbom-as-a-service]

⚠ Vulnerability Report

Please consider this as a tentative appraisal, consult Dependency-Track for reliable results.

Artifact	Vulnerability	Severity (CVSS v3)	Recommendation
pkg:maven/com.fasterxml.jackson.core/jackson-databind@2.15.3?type=jar	CVE-2023-35116 jackson-databind through 2.15.2 allows attackers to cause a denial of service or other unspecified impact via a crafted object that uses cyclic dependencies. NOTE: the vendor's perspective is that this is not a valid vulnerability report, because the steps of constructing a cyclic data structure and trying to serialize it cannot be achieved by an external attacker.	🟡 Medium (4.7)	Update to 2.16.0 or later

[heubeck]

🚀