Kubernetes env vars do not allow empty values

[antssilva96]

Describe the bug

When using quarkus kubernetes I need to set up a environment variable with value null

but I get

The config property quarkus.kubernetes.env.vars.TESTVAR is defined as the empty String ("") which the following Converter considered to be null: io.smallrye.config.Converters$BuiltInConverter -> [Help 1]

I think the problem is that in https://github.com/quarkusio/quarkus/blob/main/extensions/kubernetes/vanilla/deployment/src/main/java/io/quarkus/kubernetes/deployment/EnvVarsConfig.java the vars map is String->String and should be String->Optional if we do want this behavior to be possible.

Expected behavior

Being able to define env variables with empty values.

Actual behavior

An error prevents this.

How to Reproduce?

1. Add quarkus kubernetes dependency
2. Add to your application.properties an entry with a kubernetes env var with empty value like quarkus.kubernetes.env.vars.TESTVAR=

Output of uname -a or ver

No response

Output of java -version

openjdk version "17.0.2" 2022-01-18

GraalVM version (if different from Java)

No response

Quarkus version or git rev

<quarkus.platform.version>2.11.3.Final</quarkus.platform.version>

Build tool (ie. output of mvnw --version or gradlew --version)

Apache Maven 3.8.5

Additional information

No response

[quarkus-bot]

/cc @Sgitario, @geoand, @iocanel

[Sgitario]

It makes sense to me to allow empty values. I've already provided a fix for this: #27681