Fix support for empty docs

Author: romanowski

sbt-dotty/sbt-test/sbt-dotty/scala3doc-empty-test/build.sbt

@@ -0,0 +1 @@
+scalaVersion := sys.props("plugin.scalaVersion")

sbt-dotty/sbt-test/sbt-dotty/scala3doc-empty-test/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/sbt-dotty/scala3doc-empty-test/src/main/scala/hello/Hello.scala

@@ -0,0 +1,8 @@
+package hello
+/** Hello, world! */
+object Hello {
+  def main(args: Array[String]): Unit = {
+    val dotty: Int | String = "dotty"
+    println(s"Hello $dotty!")
+  }
+}

sbt-dotty/sbt-test/sbt-dotty/scala3doc-empty-test/test

@@ -0,0 +1,3 @@
+> doc
+> test:doc
+

sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala

@@ -444,14 +444,16 @@ object DottyPlugin extends AutoPlugin {
 
   private val docSettings = inTask(doc)(Seq(
     tastyFiles := {
-      val _ = compile.value // Ensure that everything is compiled, so TASTy is available.
+      val sources = compile.value // Ensure that everything is compiled, so TASTy is available.
       // sbt is too smart and do not start doc task if there are no *.scala files defined
       file("___fake___.scala") +:
         (classDirectory.value ** "*.tasty").get.map(_.getAbsoluteFile)
     },
     sources := Def.taskDyn[Seq[File]] {
-      if (isDotty.value && useScala3doc.value) Def.task { tastyFiles.value }
-      else Def.task { sources.value }
+      val originalSources = sources.value
+      if (isDotty.value && useScala3doc.value && originalSources.nonEmpty)
+        Def.task { tastyFiles.value }
+      else Def.task { originalSources }
     }.value,
     scalacOptions ++= {
       if (isDotty.value) {