Skip empty dataframes when creating addplots

Nobody has claimed this yet.

I add signals to my diagram using `make_addplot`. But if there are no signals in the selected time period, I get an error.

I have solved it for me as follows:

```
if stock[stock['Signal'].notnull()].shape[0] == 0:
print ("- no Signal")
else:
signal_short = mpf.make_addplot(stock['Signal'] * 0.93,
scatter=True,
markersize=150,
marker='v',
color='red'
)
add_plots.append(signal_short)
```

Would it not be an option to integrate this directly into the function and, if necessary, only issue a warning?

```
if data.notnull()].shape[0] == 0:
raise Warning("Empty Dataframe!")
```

Start at the make_addplot entry point and reproduce the issue with an empty Signal series in the selected time period. Trace how the resulting addplot is handled, then verify that empty data is skipped or reported without raising the current error. Done means the supplied no-signal case completes successfully.

Written by the indexing model from the issue text.