Fix bug on metadata XML parsing.

This fixes an issue that caused the metadata to be empty as
the code was checking if truthy of Element (XML nodes). But
those are false if they have no children.

Author: mberacochea

mg_toolkit/metadata.py

@@ -72,14 +72,14 @@ def get_metadata(self, sample_accession):
 
             # optional
             units = sample_attribute.find("UNITS")
-            if not tag:
+            if tag is None:
                 # broken metadata but not fatal
                 continue
 
             key = tag.text.strip()
             key_value = None
-            key_value = value.text.strip() if value and value.text else ""
-            if units and units.text:
+            key_value = value.text.strip() if value is not None and value.text else ""
+            if units is not None and units.text:
                 key_value += units.text.strip()
 
             return_meta[key] = key_value