Code Modernization: Replace some ternaries with null coalescing operator.

[costdev]

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Changes can be reviewed altogether or per PR commit.
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Trac ticket: https://core.trac.wordpress.org/ticket/58874

[mukeshpanchal27]

Thanks @costdev for the PR. Could you please rebased?

[costdev]

@mukeshpanchal27 This PR is on hold until a Make post has been made to establish what PHP 7 syntaxes will be supported/allowed in Core.

[westonruter]

Refresh: #10654

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