bugfix empty sequence error in periodbase
bugfix empty sequence error in periodbase

Else, when `finlsp` ahs no entries (e.g., PDM returned only infs) you
get the following ValueError:

```
File
".../astrobase-0.1.0-py3.5.egg/astrobase/periodbase/spdm.py",
line 280, in stellingwerf_pdm
  bestperiodind = npargmin(finlsp)

File
".../site-packages/numpy/core/fromnumeric.py",
line 1034, in argmin
  return argmin(axis, out)
ValueError: attempt to get argmin of an empty sequence
```

waqasbhatti/astrobase,lgbouma/astrobase,lgbouma/astrobase,waqasbhatti/astrobase,lgbouma/astrobase,waqasbhatti/astrobase