Use "= default" for constructors.

Found via: git grep '\([A-Za-z_][A-Za-z_]*\)::\1() {}'

Change-Id: I493175874e0f5ba82a069e12328dafaf16a54e82
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/75052
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/testing/range_set.cpp b/testing/range_set.cpp
index 568935d..ffd71cf 100644
--- a/testing/range_set.cpp
+++ b/testing/range_set.cpp
@@ -8,7 +8,8 @@
 
 #include "core/fxcrt/fx_system.h"
 
-RangeSet::RangeSet() {}
+RangeSet::RangeSet() = default;
+
 RangeSet::~RangeSet() = default;
 
 bool RangeSet::Contains(const Range& range) const {