Importing Pandas Tables with Missing Values

If you import a table with missing values (like below) and create a dataframe from it, everything works fine until the command is actually execute (.first(), or .toPandas(), etc). The problem came up with a much larger table with values that were not NAN, just empty.

```
import pandas as pd
from io import StringIO
test_df = pd.read_csv(StringIO(',Scan Options\n15,SAT2\n16,\n'))
sqlContext.createDataFrame(test_df).registerTempTable('Test')
o_qry = sqlContext.sql("SELECT * FROM Test LIMIT 1")
o_qry.first()
```