Verify nil objective in Experiment defaults

[andreyvelich]

Fixes: #1435.
We should verify nil objective before set the default values for metrics strategies.

/assign @gaocegege @johnugeorge
/cc @ScrapCodes

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In response to this:

Fixes: #1435.
We should verify nil objective before set the default values for metrics strategies.

/assign @gaocegege @johnugeorge
/cc @ScrapCodes

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[gaocegege]

/lgtm