Setting text to an empty string results in errors

The error in the code was this:
```
Traceback (most recent call last):
  File "code.py", line 58, in <module>
  File "code.py", line 56, in update_data
  File "/lib/adafruit_matrixportal/matrixportal.py", line 263, in set_text
  File "/lib/adafruit_display_text/label.py", line 89, in __init__
RuntimeError: Please provide a max size, or initial text
```