Prevents a potential type errors when making use of the `gmt_offset` option by casting the value to a float prior to performing calculations with the value.
This mainly accounts for incorrect storage of values, such as an empty string or city name.
Follow up to [58923].
Props chaion07, hellofromtonya, kirasong, mhshohel, mukesh27, nicolefurlan, nihar007, nurielmeni, oglekler, peterwilsoncc, prionkor, rajinsharwar, rarst, rleeson, sabernhardt, SergeyBiryukov, swissspidy, toastercookie, verygoode.
Fixes#56358, #58986, #60629.
Built from https://develop.svn.wordpress.org/trunk@59064
git-svn-id: http://core.svn.wordpress.org/trunk@58460 1a063a9b-81f0-0310-95a4-ce76da25c4cd
2cc6bb80ef