Handle sunrise and sunset being unset

For "Walluf, Germany", the sunrise and sunset variables
were being left unset resulting in:
```
./ansiweather: line 211: [: 1385212168: unary operator expected
./ansiweather: line 211: [: 1385212168: unary operator expected
```
Merge pull request #38 from joshmoore/sunrise-fix

Handle sunrise and sunset being unset