Repository: ecmwf/earthkit-data (python)
Issue: to_pandas fails for bufr when no filters argument specified
### What happened?

Running this code:

```
ds = earthkit.data.from_source("file", "synop.bufr")
r = ds.to_pandas(columns=["latitude", "longitude"])
```
results in the following error:
```
File "git/pdbufr/pdbufr/bufr_structure.py", line 616, in stream_bufr
    filters = dict(filters)
TypeError: 'NoneType' object is not iterable
```

To make it work `filters` must be specified. E.g.:

`r = ds.to_pandas(columns=["latitude", "longitude"], filters={})`

### What are the steps to reproduce the bug?

See above

### Version

0.1.3, develop

### Platform (OS and architecture)

all

### Relevant log output

_No response_

### Accompanying data

_No response_

### Organisation

_No response_
Pull request description: to_pandas fails for bufr when no filters specified
fixes #110 

Patch:
diff --git a/earthkit/data/readers/bufr.py b/earthkit/data/readers/bufr.py
index 59c7672..d305568 100644
--- a/earthkit/data/readers/bufr.py
+++ b/earthkit/data/readers/bufr.py
@@ -62,6 +62,7 @@ class BUFRReader(Reader):
         """
         import pdbufr
 
+        filters = {} if filters is None else filters
         return pdbufr.read_bufr(self.path, columns=columns, filters=filters)
 
     def __len__(self):


Test patch:
diff --git a/tests/bufr/test_bufr_convert.py b/tests/bufr/test_bufr_convert.py
new file mode 100644
index 0000000..b9c9925
--- /dev/null
+++ b/tests/bufr/test_bufr_convert.py
@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+
+# (C) Copyright 2020 ECMWF.
+#
+# This software is licensed under the terms of the Apache Licence Version 2.0
+# which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+# In applying this licence, ECMWF does not waive the privileges and immunities
+# granted to it by virtue of its status as an intergovernmental organisation
+# nor does it submit to any jurisdiction.
+#
+
+import pandas as pd
+import pytest
+
+from earthkit.data import from_source
+from earthkit.data.testing import earthkit_examples_file
+
+assert_frame_equal = pd.testing.assert_frame_equal
+
+
+@pytest.mark.parametrize("_kwargs", [{}, {"filters": {}}])
+def test_bufr_to_pandas_no_filters(_kwargs):
+    ds = from_source("file", earthkit_examples_file("temp_10.bufr"))
+
+    res = ds.to_pandas(columns=["latitude", "longitude", "WMO_station_id"], **_kwargs)
+
+    row_ref_0 = {"latitude": 67.37, "longitude": 26.63, "WMO_station_id": 2836}
+    row_ref_9 = {"latitude": 60.82, "longitude": 23.50, "WMO_station_id": 2963}
+
+    ref_0 = pd.DataFrame(row_ref_0, index=[0])
+    ref_9 = pd.DataFrame(row_ref_9, index=[9])
+
+    assert len(res) == 10
+    assert list(res.columns) == ["latitude", "longitude", "WMO_station_id"]
+    assert_frame_equal(res[0:1], ref_0[res.columns])
+    assert_frame_equal(res[9:10], ref_9[res.columns])
+
+
+def test_bufr_to_pandas_filters():
+    ds = from_source("file", earthkit_examples_file("temp_10.bufr"))
+
+    res = ds.to_pandas(
+        columns=["latitude", "longitude", "WMO_station_id"],
+        filters={"WMO_station_id": [2836, 2963]},
+    )
+
+    ref = pd.DataFrame.from_dict(
+        {
+            "latitude": [67.37, 60.82],
+            "longitude": [26.63, 23.50],
+            "WMO_station_id": [2836, 2963],
+        }
+    )
+    assert_frame_equal(res, ref[res.columns])
+
+
+if __name__ == "__main__":
+    from earthkit.data.testing import main
+
+    main()