Bug: When parsing just one month/year, app breaks

[blalop]

Because we try to concatenate just a float with the whole dataframe:

  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/blalop/Documentos/Dev/my-year-in-bbva/yearinbbva/__main__.py", line 5, in <module>
    from app import app
  File "/home/blalop/Documentos/Dev/my-year-in-bbva/yearinbbva/app.py", line 23, in <module>
    dcc.Graph(id="by-year", figure=px.bar(operations.group_by_year)),
  File "/home/blalop/Documentos/Dev/my-year-in-bbva/yearinbbva/operations.py", line 77, in group_by_year
    return self._concat_groups(_group_amount_by_year)
  File "/home/blalop/Documentos/Dev/my-year-in-bbva/yearinbbva/operations.py", line 65, in _concat_groups
    return pd.concat(
  File "/home/blalop/.local/lib/python3.10/site-packages/pandas/util/_decorators.py", line 311, in wrapper
    return func(*args, **kwargs)
  File "/home/blalop/.local/lib/python3.10/site-packages/pandas/core/reshape/concat.py", line 346, in concat
    op = _Concatenator(
  File "/home/blalop/.local/lib/python3.10/site-packages/pandas/core/reshape/concat.py", line 436, in __init__
    raise TypeError(msg)
TypeError: cannot concatenate object of type '<class 'numpy.float64'>'; only Series and DataFrame objs are valid