clang-tidy: Fix `readability-redundant-string-init` in headers #685

Split from bitcoin/bitcoin#26705 as was requested in https://github.com/bitcoin/bitcoin/pull/26705#issuecomment-1353293405.

To test this PR, consider applying a diff as follows:

--- a/src/.clang-tidy
+++ b/src/.clang-tidy
@@ -12,17 +12,9 @@ readability-redundant-declaration,
 readability-redundant-string-init,
 '
 WarningsAsErrors: '
-bugprone-argument-comment,
-bugprone-use-after-move,
-misc-unused-using-decls,
-modernize-use-default-member-init,
-modernize-use-nullptr,
-performance-for-range-copy,
-performance-move-const-arg,
-performance-unnecessary-copy-initialization,
-readability-redundant-declaration,
 readability-redundant-string-init,
 '
 CheckOptions:
  - key: performance-move-const-arg.CheckTriviallyCopyableMove
    value: false
+HeaderFilterRegex: '.'

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #bitcoin/bitcoin/26705 (clang-tidy: Check headers and fixes them by hebasto)

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review ACK c39619eeb4e615eb991143edb99e031dd562d0f6