Update tuner.py

[haifeng-jin]

Resolves #642
When no objective is specified, we just use save every epoch.

[codecov-commenter]

Codecov Report

Merging #657 (25b56be) into master (ded9aa3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 25b56be differs from pull request most recent head 2e24458. Consider uploading reports for the commit 2e24458 to get more accurate results

@@           Coverage Diff           @@
##           master     #657   +/-   ##
=======================================
  Coverage   92.42%   92.42%           
=======================================
  Files          36       36           
  Lines        2876     2879    +3     
=======================================
+ Hits         2658     2661    +3     
  Misses        218      218           

Impacted Files	Coverage Δ
keras_tuner/engine/tuner_utils.py	86.70% <100.00%> (+0.21%)	⬆️
keras_tuner/tuners/bayesian.py	95.37% <0.00%> (ø)

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