Fix undefined array key warning for date_modified [#3587391]

Problem/Motivation

When adding a date_modified_block, this message will appear:

Warning: Undefined array key "date_modified" in Drupal\wxt_library\Plugin\Block\DateModifiedBlock->blockForm() (line 137 of modules/contrib/wxt_library/src/Plugin/Block/DateModifiedBlock.php).

Proposed resolution

Set a default value for $config['date_modified'].

Remaining tasks

Implement.

User interface changes

None, except that no warning will appear.

API changes

None.

Data model changes

None.

Issue fork wxt_library-3587391

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3587391-fix-undefined-array changes, plain diff MR !11
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