Fix Core Widget Fields Not Rendering "0" Value

[Sukhendu2002]

Trac ticket: https://core.trac.wordpress.org/ticket/38133

Description

This PR fixes an issue where core widget fields fail to render when the value is "0". The problem occurs because empty() checks in widget fields treat "0" as an empty value, causing widgets to not display content when "0" is the only value.

Problem

Currently, if you set a single "0" as the value in many default widgets (e.g., widget title or content), it won't output anything due to the use of empty() checks. The issue arises because empty() returns TRUE for "0" values, treating them as empty.

For example:

• Setting a widget title to "0" results in no title being displayed
• Multiple zeros ("00") or other values work correctly
• This affects multiple default widgets where empty() is used for validation

Solution

Replace empty() checks with explicit isset() and string comparison:

// Before
if ( ! empty( $instance['title'] ) ) {
    $title = $instance['title'];
}

// After
if ( isset( $instance['title'] ) && '' !== $instance['title'] ) {
    $title = $instance['title'];
}

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