Helm plugin fails when no sources are found

[imanushin]

Steps to reproduce:

1. Create a project
2. Don't add any sources under src/main/helm
3. Run helmPackage (actually, or any other helm-specific task).

Expected result:
Plugin fails with error like "no charts are defined and no sources were found under the folder ..."

Actual result:
the following error appears:

A problem was found with the configuration of task ':helmFilterMainChartSources' (type 'HelmFilterSources').
  - In plugin 'com.citi.gradle.plugins.helm.command.HelmDownloadClientPlugin' type 'com.citi.gradle.plugins.helm.tasks.HelmFilterSources' property 'sourceDir' doesn't have a configured value.
    
    Reason: This property isn't marked as optional and no value has been configured.
    
    Possible solutions:
      1. Assign a value to 'sourceDir'.
      2. Mark property 'sourceDir' as optional.
    
    For more information, please refer to https://docs.gradle.org/8.4/userguide/validation_problems.html#value_not_set in the Gradle documentation.