AppendingTransformer generates undesired empty lines that cause errors

### 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.

Start by locating the AppendingTransformer implementation and reproduce the merge using META-INF/maven/org.apache.maven.api.di.Inject. Check how line endings are handled between merged files; done means the output contains no undesired empty entries and maven-di no longer fails while loading it.

Written by the indexing model from the issue text.