add unit test to check empty tracer

tests/test_backtrace.cpp

@@ -31,6 +31,19 @@ TEST_CASE("bactrace1", "[bactrace]")
     REQUIRE(test_sink->lines()[7] == "****************** Backtrace End ********************");
 }
 
+TEST_CASE("bactrace-empty", "[bactrace]")
+{
+    using spdlog::sinks::test_sink_st;
+    auto test_sink = std::make_shared<test_sink_st>();
+    size_t backtrace_size = 5;
+
+    spdlog::logger logger("test-backtrace", test_sink);
+    logger.set_pattern("%v");
+    logger.enable_backtrace(backtrace_size);
+    logger.dump_backtrace();
+    REQUIRE(test_sink->lines().size() == 0);
+}
+
 TEST_CASE("bactrace-async", "[bactrace]")
 {
     using spdlog::sinks::test_sink_mt;