fix: Ignore trials without metrics

[gaocegege]

Signed-off-by: Ce Gao gaoce@caicloud.io

What this PR does / why we need it:

Some metrics will be ignored if it is none.

ERROR:grpc._server:Exception calling application: 'str' object has no attribute 'value'
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/grpc/_server.py", line 434, in _call_behavior
    response_or_iterator = behavior(argument, context)
  File "/usr/src/app/github.com/kubeflow/katib/pkg/suggestion/v1alpha3/hyperopt_service.py", line 27, in GetSuggestions
    search_space, trials, request.request_number)
  File "/usr/src/app/github.com/kubeflow/katib/pkg/suggestion/v1alpha3/hyperopt/base_hyperopt_service.py", line 95, in getSuggestions
    objective_for_hyperopt = float(trial.target_metric.value)
AttributeError: 'str' object has no attribute 'value'

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

---

This change is 

[johnugeorge]

/lgtm

[johnugeorge]

/approve

[k8s-ci-robot]

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