minor #28025 Use an empty string as default for assertEquals (DavidPr…

…evot)

This PR was merged into the 3.4 branch.

Discussion
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Use an empty string as default for assertEquals

Make sure it isn’t interpreted as a type NULL, making the test fail with PHPUnit 7.2.

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

This shouldn’t break the existing tests, but will also make them work with recent PHPUnit.

Commits
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31353147a3 Use an empty string as default for assertEquals

Tests/Fixtures/escapedCharacters.yml

@@ -4,7 +4,7 @@ yaml: |
 php: |
     "\\0 \\ \\a \\b \\n"
 ---
-test: null
+test: 'null'
 yaml: |
     "\0"
 php: |