Fix for an occasional crash in the P&L Report

[bug-or-feature]

This can happen when you have moved to a new strategy, and there are instruments for which there have been no trades yet.

P&L data for an instrument in a strategy is generated by first building a series of prices at merge_fill_prices_with_prices() from systems.accounts.pandl_calculators.pandl_using_fills.py. A series of P&L values is built from the prices, and then the P&L series are later concatenated into a dataframe at get_df_of_perc_pandl_series_for_strategy_all_instruments() in sysproduction.reporting.data.pandl.py

When there are no trades yet for the new strategy, the empty series of prices has an index of dtype object. That causes the P&L dataframe to be corrupted, only containing data for the final instrument. That causes the crash.

The fix is to set the index to be of datetime type, even for an empty series

Also changes the pandl module print() statements to log.info()