F fix merge key word

Summary

It was possible that the merge_entities function ran into a KeyError. This is now fixed.

Focus

The first commit adds a failing test. The second commit introduces the fix.

Test Environment

unittest

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src/caosdb/apiutils.py

@@ -338,11 +338,13 @@ def merge_entities(entity_a: Entity, entity_b: Entity):
                 raise NotImplementedError()
 
             for attribute in ("datatype", "unit", "value"):
-                if diff_r1["properties"][key][attribute] is None:
-                    setattr(entity_a.get_property(key), attribute,
-                            diff_r2["properties"][key][attribute])
-                else:
-                    raise RuntimeError("Merge conflict.")
+                if (attribute in diff_r2["properties"][key] and
+                        diff_r2["properties"][key][attribute] is not None):
+                    if (diff_r1["properties"][key][attribute] is None):
+                        setattr(entity_a.get_property(key), attribute,
+                                diff_r2["properties"][key][attribute])
+                    else:
+                        raise RuntimeError("Merge conflict.")
         else:
             # TODO: This is a temporary FIX for
             #       https://gitlab.indiscale.com/caosdb/src/caosdb-pylib/-/issues/105