More tests.

Author: trivialfis

src/common/common.h

@@ -79,7 +79,6 @@ namespace xgboost::common {
   if (str.empty()) {
     return str;
   }
-
   auto first = str.find_first_not_of(" \t\n\r");
   if (first == std::string_view::npos) {
     return {};

tests/cpp/common/test_common.cc

@@ -44,6 +44,18 @@ TEST(Common, Trim) {
     auto res = TrimLast(" foo ");
     ASSERT_EQ(res, std::string_view{" foo"});
   }
+  {
+    auto res = TrimLast("  ");
+    ASSERT_EQ(res, std::string_view{});
+  }
+  {
+    auto res = TrimFirst("  ");
+    ASSERT_EQ(res, std::string_view{});
+  }
+  {
+    auto res = TrimFirst("");
+    ASSERT_EQ(res, std::string_view{});
+  }
 }
 
 TEST(Common, Split) {