Are you sure want to not remove empty environment variables?
What are your use cases?

We need to blank a variable (a perforce label) based on certain combinations of user input.

Another example: We need to add maven profiles only certain combinations of user input.
In the example below ${MAVEN_PROPERTIES} and ${MAVEN_PROFILES} need to be blank under certain circumstances.

Build:
Goals and options: ${MAVEN_GOALS} ${MAVEN_PROPERTIES} ${MAVEN_PROFILES}

Code changed in jenkins
User: Gregory Boissinot
Path:
src/main/java/org/jenkinsci/plugins/envinject/service/EnvInjectEnvVars.java
src/test/java/org/jenkinsci/plugins/envinject/EnvInjectEnvVarsEmpty.java
http://jenkins-ci.org/commit/envinject-plugin/718586449693a86314242223b14813c374b8bbdc
Log:
Fix JENKINS-15146

Sorry for the inconvenience.
Empty environment variables are acceptable for now.

I'm getting the same behaviour (blank properties are not set) in 1.92.1

it's still not possible to have an empty Environment variable.
Using pipeline step withEnv(['FOO=']) with result in key="FOO" and value="".
Then EnvVars.override is called and even removes "FOO" from Environment.

https://github.com/kohsuke/hudson/blob/master/core/src/main/java/hudson/EnvVars.java#L126

if(value==null || value.length()==0) {
    remove(key);
    return;
}

The behavior would be okay, if the value is null. But for an explicit empty String it seems not reasonable, that the Key is deleted.

What needs to be done to change this? It could not be done in the withEnv Code except in dirty ways.

we use it for parameters to give the starter of a job the possibility to extend an existing path with a subpath. if the starter wants to use the root he wont type anything for this parameter... this doesn't work anymore...

think Pipeline does not use EnvInject so any Pipeline bug should be filed separately with steps to reproduce from scratch, in workflow-plugin.

I want to set a environment variable to an empty string ("") via withEnv(), but it seems not working.