AppendingTransformer generates undesired empty lines that cause errors

[jafarre-bi]

Affected version

3.6.2

Bug description

I'm transforming the files META-INF/maven/org.apache.maven.api.di.Inject using the AppendingTransformer.
After each merged file, it adds an empty line. From what I see in the code, it blindly adds a line feed after appending each file's contents, so it generates an empty line for each line ending with a line feed. Unfortunately, maven-di tries to load empty entries, resulting in exceptions. This makes it unusable in my use case, and I don't have a workaround.

[jafarre-bi]

While writing a PR to fix this "bug", I realised it is documented behaviour. However, I think adding an opt-out for the newline separator allows us to use the AppendingTransformer in situations that disallow empty lines, like in my use case.
I'd like to provide a PR with this opt-out. Any rules or guidelines I should take into account? Thank you.

[jafarre-bi]

PR #802 adds the opt-out

[cstamas]

@gnodet I think MavenDI should just ignore empty (and commented out?) lines?

@jafarre-bi Care to report this as Maven issue?

[jafarre-bi]

@cstamas Sure thing. Yet, I think this opt-out is a good idea. It's a very simple change that can benefit other use cases where empty lines may cause problems.

[jafarre-bi]

Related issue: apache/maven#11827