[imp] Added a validation message for the URL field when schema is not provided

[marrouchi]

Pull Request for Issue #13870.

Summary of Changes

Added a validation message to the JFormRuleUrl class.

Testing Instructions

1. Create a url field : fill schema and make sure it's not relative
2. Create a new content and fill the url field with a domain name (example.com).

Expected result

A warning message displays "Invalid URL : URL schema is missing in example.com. Please add one of the following at the begining : http...." with no explanation.

Actual result

A warning message displays "Invalid field" with no explanation.

Documentation Changes Required

No.

[ghost]

I have tested this item ✅ successfully on c0048b4

Lang-String missing:

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[justinherrin]

Just a minor spelling typo to correct...
"Invalid URL : URL schema is missing in %s. Please add one of the following at the begining : %s."

Should be...
"Invalid URL : URL schema is missing in %s. Please add one of the following at the beginning : %s."

[ghost]

I have tested this item ✅ successfully on c1636c2

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[infograf768]

Please use ordered variables in the string
"Invalid URL : URL schema is missing in %1$s. Please add one of the following at the beginning : %2$s ."

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13885.}

[infograf768]

I have tested this item ✅ successfully on 19a0e9f

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[marrouchi]

RTC please

[infograf768]

RTC as the change in the string concerns only the type of variable.

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