Add error in debug-data if no dev docs are available (see #4575)

spacy/cli/debug_data.py

@@ -121,6 +121,8 @@ def debug_data(
     msg.text("{} training docs".format(len(train_docs)))
     msg.text("{} evaluation docs".format(len(dev_docs)))
 
+    if not len(dev_docs):
+        msg.fail("No evaluation docs")
     overlap = len(train_texts.intersection(dev_texts))
     if overlap:
         msg.warn("{} training examples also in evaluation data".format(overlap))