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/add_remove.rs

@@ -4,10 +4,7 @@ use web_sys::{console, wasm_bindgen::JsValue};
 use crate::{
     engine,
     AppState,
-    assembly::{
+    assembly::{Assembly, Element}
-        Assembly,
-        Element
-    }
 };
 
 /* DEBUG */

app-proto/src/outline.rs

@@ -9,11 +9,7 @@ use web_sys::{
 use crate::{
     AppState,
     assembly,
-    assembly::{
+    assembly::{ElementKey, Regulator, RegulatorKey},
-        ElementKey,
-        Regulator,
-        RegulatorKey
-    },
     specified::SpecifiedValue
 };
 

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) }