Check for null value with the appropriate assertion method

[valery1707]

Description

Replace all occurrences of Assert.isTrue\((\w+)\s?!=\s?null\s*, with Assert.notNull($1,.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Changes would cause existing functionality to not work as expected
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[CLAassistant]

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