Also add spoon task for empty sourceSets

[skiedrowski]

SpoonPlugin#apply skips adding spoon tasks if source.empty. If the source set is populated after configuration phase by some sort of code generation, this might not be what we want.

Example

• sourceSet X is empty (or the directory does not even exist)
• gradle assemble
  • configuration phase -> SpoonPlugin decides to skip sourceSet X
  • code generation adds files to sourceSet X
  • compileJavaX compiles sources of sourceSet X (without prior spooning)

As a workaround, the build needs to make sure that the sourceSet is not empty during configuration phase.

[mictaege]

The idea of skipping "empty" sourcesets was to avoid unnecessary work during the build. In practice, this optimization should bring little or no performance at all.

So in this regard, we can confidently do without it.

The only thing we have to keep in mind is that the source code transformation with Spoon will not fail if it is a really empty sourceset, where no source code exists and no generation take place.