Fix wp core deprecation messages for strpos and str_replace

[Imran92]

Resolves #7016

Proposed Changes

• Multiple deprecation messages are appearing in PHP 8.1 and 8.2. Among those messages, 6 were related to strpos and str_replace. This PR fixes those.

Though the error message shows that it's being thrown from the core (wp-includes/functions.php), these are actually due to some calls to the function add_submenu_page in Sensei core.

Among the parameters we use for calling add_submenu_page, the first one for the parent. If it's not set, the page is added but it's not listed under the visible menu. So use use empty string or null as the first parameter for some pages, like the course and lesson ordering pages.

Sending empty strings is alright, but sending null actually makes core wp throw a deprecation message because sending null to the haystack for strpos is no longer allowed, which is called internally by the add_submenu_page function. So we are changing the nulls to empty strings here. And in the future, we should not use null anymore in similar use case.

Testing Instructions

• Use PHP 8.1 or 8.2 at least
• Set WP_DEBUG, WP_DEBUG_DISPLAY, WP_DEBUG_LOG to true
• Load the dashboard
• Make sure you see no deprecation messages visible in the rendered page or the log

Note

For PHP 8.2, you'll still see some deprecation messages regarding dynamically created variables. Those will be fixed separately under this issue, because it's more specific to that deprecation message.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7028 (fa0899a) into trunk (689f545) will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##              trunk    #7028   +/-   ##
=========================================
  Coverage     48.03%   48.03%           
  Complexity    10458    10458           
=========================================
  Files           573      573           
  Lines         44141    44141           
  Branches        402      402           
=========================================
  Hits          21201    21201           
  Misses        22613    22613           
  Partials        327      327           

Impacted Files	Coverage Δ
includes/class-sensei-admin.php	19.08% <0.00%> (ø)
includes/class-sensei-course.php	38.49% <0.00%> (ø)
includes/class-sensei-modules.php	36.62% <0.00%> (ø)

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[donnapep]

Small mistake in the change log entry