remove std::experimental usage

All of the std::experimental usages in this repository have a well
supported counterpart in std as of C++17.  Switch to use them.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4cdf5cd27053ce85c7f70c215ee9456f96f79f42
diff --git a/src/journal.cpp b/src/journal.cpp
index 41d51e4..09f98aa 100644
--- a/src/journal.cpp
+++ b/src/journal.cpp
@@ -32,7 +32,7 @@
         const auto& storage = std::get<storageIndex>(n.second);
         const auto& value = std::get<valueIndex>(storage.get());
 
-        if (!value.empty())
+        if (value.has_value())
         {
             log(message, pathMeta, path, propertyMeta, value);
         }