MAINT: Bail out more gracefully if profile data isn't present

[HaoZeke]

Currently loading a benchmark without profile data present and checking for it raises, which could well be a user warning instead.

File ~/Git/Github/asvWork/asv/asv/results.py:564, in Results.get_profile(self, benchmark_name)
    549 """
    550 Get the profile data for the given benchmark name.
    551 
   (...)
    561 
    562 """
    563 profile_data = self._profiles[benchmark_name]
--> 564 profile_data = profile_data.encode('ascii')
    565 profile_bytes = zlib.decompress(base64.b64decode(profile_data))
    566 return profile_bytes

AttributeError: 'NoneType' object has no attribute 'encode'

In [64]: b1.get_profile('benchmarks.TimeSuite.time_add_arr')

Doesn't really matter but still, a better error / warning would be nicer.