Consolidate constructions from empty specification

To reduce the potential for inconsistency, we should only have one piece
of code that constructs a `SpecifiedValue` from the empty specification.

app-proto/src/specified.rs

@@ -34,7 +34,7 @@ impl TryFrom<String> for SpecifiedValue {
     
     fn try_from(spec: String) -> Result<Self, Self::Error> {
         if spec.is_empty() {
-            Ok(SpecifiedValue { spec: spec, value: None })
+            Ok(SpecifiedValue::from_empty_spec())
         } else {
             spec.parse::<f64>().map(
                 |value| SpecifiedValue { spec: spec, value: Some(value) }