eval, repr, and empty Instruments instantiation error

[rstoneback]

Describe the bug
Empty instrument object doesn't have a string representation that leads back to itself.

To Reproduce
Steps to reproduce the behavior:

In [1]: import pysat                                                                                                                                                                                                        

In [2]: inst = pysat.Instrument()                                                                                                                                                                                           

In [3]: repr(inst)                                                                                                                                                                                                          
Out[3]: "pysat.Instrument(platform='', name='', tag='', inst_id='', clean_level='clean', pad=None, orbit_info={}, inst_module=None, custom=[], **{})"

In [4]: eval(repr(inst))                                                                                                                                                                                                    
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-4-c961af2d6bbf> in <module>
----> 1 eval(repr(inst))

<string> in <module>

~/Code/pysat/pysat/_instrument.py in __init__(self, platform, name, tag, inst_id, clean_level, update_files, pad, orbit_info, inst_module, directory_format, file_format, temporary_file_list, strict_time_flag, ignore_empty_files, labels, custom, **kwargs)
    263 
    264                 # Look to module for instrument functions and defaults
--> 265                 self._assign_attrs(by_name=True, tag=self.tag,
    266                                    inst_id=self.inst_id)
    267             elif (platform is None) and (name is None):

~/Code/pysat/pysat/_instrument.py in _assign_attrs(self, by_name, inst_module, tag, inst_id)
   1210                 user_modules = pysat.params['user_modules']
   1211                 if self.platform not in user_modules.keys():
-> 1212                     raise KeyError('unknown platform supplied: {:}'.format(
   1213                         self.platform))
   1214 

KeyError: 'unknown platform supplied: '

Expected behavior
Another empty Instrument object like we were able to instantiate to begin with.

Screenshots
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Desktop (please complete the following information):

• OS: MacOS
• Version Python 3.8
• Other details about your setup that could be relevant

Additional context
Add any other context about the problem here.

[jklenzing]

Requires merge of #903 since new instrument tests will be added

[jklenzing]

For a blank instrument, self._assign_attrs should be called as a default function. After instantiation, instruments with platform=None will be reset to platform=''. This triggers self_assign_attrs(by_name=True) on subsequent runs, such as when eval is called. This is the source of the errors, as multiple checks are run under by_name=True that assume a module must be present under inst_module.

[jklenzing]

Closing with merge of #918