Fix ISH for empty sites

[zmoon]

Discovered some bugs related to empty sites while setting up ISH support for MM.

• for partially empty (i.e. one or more but not all sites missing all data for that period), bytes conversion to string would fail, now the method just returns the input df if empty
• for fully empty, the resample groupby would fail, now skipped if empty

[zmoon]

Note EPA AQS link check was briefly failing is failing intermittently

( index: line 73) broken https://www.epa.gov/aqs/ - HTTPSConnectionPool(host='origin-awswest-www.epa.gov', port=443): Max retries exceeded with url: /aqs (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7f04b087e880>: Failed to establish a new connection: [Errno 110] Connection timed out'))

usually it succeeds if I just run it a second time though, weird