Use overloaded function for getFromLocalStorage type

This allows clients to not provide a default value, in which case
undefined is returned they must explicitly handle it.

Also move a CSS import from App.tsx to index.tsx

src/App.tsx

@@ -1,4 +1,3 @@
-import "currency-flags/dist/currency-flags.css";
 import { FunctionComponent, useEffect, useReducer, useState } from "react";
 import { BalanceContainer } from "./components/BalanceContainer";
 import { ShortcutsContainer } from "./components/ShortcutsContainer";

src/index.tsx

@@ -1,8 +1,10 @@
 import { StrictMode } from "react";
 import { render } from "react-dom";
-import "./index.scss";
 import { App } from "./App";
 
+import "./index.scss";
+import "currency-flags/dist/currency-flags.css";
+
 render(
     <StrictMode>
         <App />

src/util/LocalStorageHelper.ts

@@ -5,16 +5,20 @@ export enum LocalStorageKey {
     DISPLAYED_BALANCES = "displayedBalances",
 }
 
-export const getFromLocalStorage = (key: LocalStorageKey, defaultValue: string | (() => string) = ""): string => {
+export function getFromLocalStorage(key: LocalStorageKey): string | undefined;
+export function getFromLocalStorage(key: LocalStorageKey, defaultValue: string): string;
+export function getFromLocalStorage(key: LocalStorageKey, defaultValue?: string): string | undefined {
     let value = localStorage.getItem(key);
-    if (value === null) {
-        if (typeof defaultValue === "string") {
-            return defaultValue;
-        }
-        return defaultValue();
+    if (value) {
+        return value;
     }
-    return value;
-};
+
+    if (typeof defaultValue === "string") {
+        return defaultValue;
+    }
+
+    return undefined;
+}
 
 export const setLocalStorage = (key: LocalStorageKey, value: string) => {
     localStorage.setItem(key, value);