[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)