Selective generation problem

Description

The use of the selective generation seems doesn't work properly.
For example using the directive "-Dmodels=User,Pet" generates only User and not Pet.

Swagger-codegen version

swagger-codegen-cli-2.3.1

Command line used for generation

example for java, but I think it is a common problem.

java -jar swagger-codegen-cli-2.3.1.jar generate --input-spec myoas.yaml --output c:\Users/clients/java --lang java --group-id com.mygroup --artifact-id logging --artifact-version 1.0.0-SNAPSHOT --api-package com.mygroup.logging.api --invoker-package com.mygroup.logging.invoker --model-package com.mygroup.logging.models -Djava8=true -DdateLibrary=java8 -DhideGenerationTimestamp=true -Dmodels=User,Pet

Suggest a fix/enhancement

Probably the problem is in OptionUtils.parseCommaSeparatedTuples() that for the -Dmodels=User,Pet produces a list with 2 entries:
1- models->User
2- Pet->""

Start in OptionUtils.parseCommaSeparatedTuples(), identified in the issue, and reproduce the behavior with the supplied Java CLI command using -Dmodels=User,Pet. Verify that selective generation produces both User and Pet rather than only User.