[DomCrawler] Fixed the coding standards to use strict comparisons
minor #10928 [DomCrawler] Fixed the coding standards to use strict comparisons (stof)

This PR was merged into the 2.3 branch.

Discussion
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[DomCrawler] Fixed the coding standards to use strict comparisons

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Many places in DomCrawler are using loose comparison instead of strict comparison. I saw them while checking the whole component for DOMNode vs DOMElement usage whe working on #10927. However, I submitted this change to 2.3 instead, to ease merging between branches (applying the change only in master would likely create conflicts regularly when merging changes in DomCrawler between branches later)

Commits
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77b446c [DomCrawler] Fixed the coding standards to use strict comparisons