Calling wptexturize() with trailing less-than symbol causes PHP warning

[westonruter]

Calling wptexturize( 'foo< ) causes a PHP warning:

Uninitialized string offset: 1

The issue lies in the _wptexturize_pushpop_element function which is not checking if an array item is set. It needs this change:

- if ( '/' !== $text[1] ) {
+ if ( isset( $text[1] ) && '/' !== $text[1] ) {

Discovered while testing #27355.

[ocean90]

In 36578:

Formatting: Avoid a PHP warning when wptexturize() is called with a trailing less-than symbol.

Props westonruter.
Fixes #35864.