Honor falsey custom data processors

fitparse/base.py

@@ -582,7 +582,8 @@ class DataProcessorMixin:
     """Add data processing to the FitFileDecoder"""
 
     def __init__(self, *args, **kwargs):
-        self._processor = kwargs.pop("data_processor", None) or FitFileDataProcessor()
+        processor = kwargs.pop("data_processor", None)
+        self._processor = FitFileDataProcessor() if processor is None else processor
         super().__init__(*args, **kwargs)
 
     def _parse_data_message(self, header):

tests/test.py

@@ -7,7 +7,7 @@
 import warnings
 
 from fitparse import FitFile
-from fitparse.processors import UTC_REFERENCE, StandardUnitsDataProcessor
+from fitparse.processors import FitFileDataProcessor, UTC_REFERENCE, StandardUnitsDataProcessor
 from fitparse.records import BASE_TYPES, Crc
 from fitparse.utils import FitEOFError, FitCRCError, FitHeaderError, FitParseError
 
@@ -74,6 +74,22 @@ def testfile(filename):
 
 
 class FitFileTestCase(unittest.TestCase):
+    def test_uses_falsey_custom_data_processor(self):
+        class FalseyProcessor(FitFileDataProcessor):
+            def __init__(self):
+                self.called = False
+
+            def __bool__(self):
+                return False
+
+            def run_message_processor(self, data_message):
+                self.called = True
+
+        processor = FalseyProcessor()
+        FitFile(testfile('activity-settings.fit'), data_processor=processor).parse()
+
+        self.assertTrue(processor.called)
+
 
     def test_basic_file_with_one_record(self, endian='<'):
         f = FitFile(generate_fitfile(endian=endian))