Make sure the queried object is non-null before accessing its properties.

Props markoheijnen, ryan.
Fixes #21394.


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git-svn-id: http://core.svn.wordpress.org/trunk@25272 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add an action at the end of `WP_Query::parse_tax_query()` called (wait for it...) `parse_tax_query` which will enable developers to alter `$this->tax_query->queries` before SQL is generated. `pre_get_posts` is too early for many scenarios.


Built from https://develop.svn.wordpress.org/trunk@25311


git-svn-id: http://core.svn.wordpress.org/trunk@25273 1a063a9b-81f0-0310-95a4-ce76da25c4cd