clarified Console_Progress.echo: include empty lines as in other Progress instances, especially relevant for Progress.bash (e.g. "isabelle phabricator ./bin/config help");

--- a/src/Pure/General/output.scala	Mon Nov 11 17:32:40 2019 +0100
+++ b/src/Pure/General/output.scala	Tue Nov 12 16:02:29 2019 +0100
@@ -21,25 +21,25 @@
   def error_message_text(msg: String): String =
     cat_lines(split_lines(clean_yxml(msg)).map("*** " + _))
 
-  def writeln(msg: String, stdout: Boolean = false)
+  def writeln(msg: String, stdout: Boolean = false, include_empty: Boolean = false)
   {
-    if (msg != "") {
+    if (msg.nonEmpty || include_empty) {
       if (stdout) Console.print(writeln_text(msg) + "\n")
       else Console.err.print(writeln_text(msg) + "\n")
     }
   }
 
-  def warning(msg: String, stdout: Boolean = false)
+  def warning(msg: String, stdout: Boolean = false, include_empty: Boolean = false)
   {
-    if (msg != "") {
+    if (msg.nonEmpty || include_empty) {
       if (stdout) Console.print(warning_text(msg) + "\n")
       else Console.err.print(warning_text(msg) + "\n")
     }
   }
 
-  def error_message(msg: String, stdout: Boolean = false)
+  def error_message(msg: String, stdout: Boolean = false, include_empty: Boolean = false)
   {
-    if (msg != "") {
+    if (msg.nonEmpty || include_empty) {
       if (stdout) Console.print(error_message_text(msg) + "\n")
       else Console.err.print(error_message_text(msg) + "\n")
     }

--- a/src/Pure/System/progress.scala	Mon Nov 11 17:32:40 2019 +0100
+++ b/src/Pure/System/progress.scala	Tue Nov 12 16:02:29 2019 +0100
@@ -60,7 +60,7 @@
 class Console_Progress(verbose: Boolean = false, stderr: Boolean = false) extends Progress
 {
   override def echo(msg: String): Unit =
-    Output.writeln(msg, stdout = !stderr)
+    Output.writeln(msg, stdout = !stderr, include_empty = true)
 
   override def theory(theory: Progress.Theory): Unit =
     if (verbose) echo(theory.message)