question: filling missing values in one column with values from another column
Is there a more concise way of accomplishing the following?

```
portfolio_data[FIELD_NAME_COUNTRY] = portfolio_data[FIELD_NAME_ISSUER_COUNTRY]
countries = portfolio_data[FIELD_NAME_COUNTRY]
flag_no_country = countries.isnull()
countries[flag_no_country] = portfolio_data.ix[flag_no_country, FIELD_NAME_MSCI_COUNTRY]
portfolio_data[FIELD_NAME_COUNTRY] = countries
```
