GABC converter: crash on empty header value

```python
from music21 import converter
import chant21

gabc = """name: Empty header value MVP;
annotation:;
%%
(c4) la(h)"""

converter.parse('gabc: ' + gabc)
```

results in `arpeggio.NoMatch: Expected attribute_value at position (2, 12) => 'nnotation:*; %% (c4) '.`

The same happens if a header value consists solely of one or more spaces.