fix: skip void methods (fixes #555)

[YanivEidelnantDev]

@JsonUnwrapped when paired with @Setter or @Data would generate a setter which has a void return value which caused this error:

Cannot invoke "com.fasterxml.classmate.ResolvedType.getTypeParameters()" because "javaType" is null 

this should fix it

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[YanivEidelnantDev]

@CarstenWickner Hi. Any updates?
It's a simple fix it would help me a lot in a release version.

[YanivEidelnantDev]

@CarstenWickner Hello again asking for a review for this.