Fixed workspace list error message

If no workspaces were found, bst workspace would return `[]`
This has been changed to a nicer message

buildstream/_stream.py

diff --git a/buildstream/_stream.py b/buildstream/_stream.py
index f4661cc..70392f5 100644
--- a/buildstream/_stream.py
+++ b/buildstream/_stream.py
@@ -641,6 +641,9 @@
             }
             workspaces.append(workspace_detail)
 
+        if not workspaces:
+            workspaces = "No workspaces found"
+
         _yaml.dump({
             'workspaces': workspaces
         })

tests/frontend/cross_junction_workspace.py

diff --git a/tests/frontend/cross_junction_workspace.py b/tests/frontend/cross_junction_workspace.py
index eb2bc2e..bd6823c 100644
--- a/tests/frontend/cross_junction_workspace.py
+++ b/tests/frontend/cross_junction_workspace.py
@@ -93,9 +93,10 @@
     result.assert_success()
 
     loaded = _yaml.load_data(result.output)
-    assert isinstance(loaded.get('workspaces'), list)
-    workspaces = loaded['workspaces']
-    assert len(workspaces) == 0
+    if not loaded['workspaces'] == "No workspaces found":
+        assert isinstance(loaded.get('workspaces'), list)
+        workspaces = loaded['workspaces']
+        assert len(workspaces) == 0
 
 
 def test_close_all_cross_junction(cli, tmpdir):
@@ -112,6 +113,7 @@
     result.assert_success()
 
     loaded = _yaml.load_data(result.output)
-    assert isinstance(loaded.get('workspaces'), list)
-    workspaces = loaded['workspaces']
-    assert len(workspaces) == 0
+    if not loaded['workspaces'] == "No workspaces found":
+        assert isinstance(loaded.get('workspaces'), list)
+        workspaces = loaded['workspaces']
+        assert len(workspaces) == 0