meson: fix new warnings found by clang-tidy

meson/clang-tidy found a few more warnings
- unused variables
- braces around scalar initializer

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I580b2603fb20dcfd2627a4e609d6052bcf38a357
diff --git a/systemd_target_signal.cpp b/systemd_target_signal.cpp
index d85f819..f27e418 100644
--- a/systemd_target_signal.cpp
+++ b/systemd_target_signal.cpp
@@ -60,7 +60,7 @@
             return (&targetEntry->second.errorToLog);
         }
     }
-    return {nullptr};
+    return nullptr;
 }
 
 void SystemdTargetLogging::systemdUnitChange(sdbusplus::message::message& msg)