fix: prevent "Undefined index: post_type" notice

[ravinderk]

Bug Report

User Story

As an admin, I do not want any notice related to Give core.

Current Behavior

In few cases, if somehow post_type will not set in WordPress global query then we will get PHP notice.
https://github.com/impress-org/give/blob/b6b83dd7bab9e8066d9399bb40186dc642d8a817/includes/admin/forms/dashboard-columns.php#L468

Expected Behavior

The plugin must load without any notice.

Bug Type

• This bug describes functionality that once worked as expected in version X.X.X.
• This bug describes functionality that never worked as expected.
• I am not sure whether this functionality ever worked as expected.

Possible Solution

Use empty to check whether this param exist or not

Related

https://wordpress.org/support/topic/error-cannot-modify-header-information-14/
https://wordpress.org/support/topic/php-notice-undefined-index-post_type/

Acceptance Criteria

• Plugin must load without any notice if post_type does not set.