Ensure match pattern error isn't raised on empty errors (closes #3549)

spacy/matcher/matcher.pyx

@@ -105,7 +105,7 @@ cdef class Matcher:
                 raise ValueError(Errors.E012.format(key=key))
             if self.validator:
                 errors[i] = validate_json(pattern, self.validator)
-        if errors:
+        if any(err for err in errors.values()):
             raise MatchPatternError(key, errors)
         key = self._normalize_key(key)
         for pattern in patterns:

spacy/tests/regression/test_issue3549.py

@@ -0,0 +1,15 @@
+# coding: utf8
+from __future__ import unicode_literals
+
+import pytest
+from spacy.matcher import Matcher
+from spacy.errors import MatchPatternError
+
+
+def test_issue3549(en_vocab):
+    """Test that match pattern validation doesn't raise on empty errors."""
+    matcher = Matcher(en_vocab, validate=True)
+    pattern = [{"LOWER": "hello"}, {"LOWER": "world"}]
+    matcher.add("GOOD", None, pattern)
+    with pytest.raises(MatchPatternError):
+        matcher.add("BAD", None, [{"X": "Y"}])