freqAI bug causing failures on 2nd backtest

[xmatthias]

Summary

if we don't call freqai_set_targets() - we can't get .find_labels() either (nested in get_unique_classes_from_labels() - as there won't be targets assigned to the dataframe.
This overwrites the data loaded from the metadata file to dk.unique_class_list and dk.label_list - causing them to be empty - and subsequently, failures like the following:

  File freqtrade/freqai/freqai_interface.py", line 901, in backtesting_fit_live_predictions        
    dk.full_df.at[index, f"{label}_mean"] = self.dk.data["labels_mean"][                                                                     
                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                     
KeyError: '&-target'  

closes #10997

Quick changelog

• update indentation to not run get_unique_classes_from_labels() if set_freqai_targets() hasn't been ran.

[coveralls]

coverage: 94.397% (+0.009%) from 94.388%
when pulling a619968 on fix/freqai_bug
into 9ff4298 on develop.