Description:
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A newline is added to the mail headers when mail.add_x_header is used and no other headers are passed to mail().

The scenario in which custom headers are used was already fixed in #48620, back in 2009; this is different.

Test script:
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assert(ini_get('mail.add_x_header') == 1);
mail("me@example.com", "Test", "A line of text.");

Expected result:
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From: me@mymachine.local (Me)
X-PHP-Originating-Script: 501:-

A line of text.

Actual result:
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From: me@mymachine.local (Me)
X-PHP-Originating-Script: 501:-


A line of text.