clang-tidy, qt: Fix modernize-use-default-member-init in headers

See https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-default-member-init.html

src/qt/qvalidatedlineedit.cpp

@@ -7,10 +7,8 @@
 #include <qt/bitcoinaddressvalidator.h>
 #include <qt/guiconstants.h>
 
-QValidatedLineEdit::QValidatedLineEdit(QWidget *parent) :
-    QLineEdit(parent),
-    valid(true),
-    checkValidator(nullptr)
+QValidatedLineEdit::QValidatedLineEdit(QWidget* parent)
+    : QLineEdit(parent)
 {
     connect(this, &QValidatedLineEdit::textChanged, this, &QValidatedLineEdit::markValid);
 }