ProgressBar null not undefined (#6953)

* ProgressBar null not undefined

* notification banner null

* update progress dialog

* docs: update type

---------

Co-authored-by: Jesse Mazzella <ozyx@users.noreply.github.com>
Co-authored-by: Jesse Mazzella <jesse.d.mazzella@nasa.gov>

src/ui/layout/BrowseBar.vue

@@ -359,7 +359,7 @@ export default {
     },
     saveAndFinishEditing() {
       let dialog = this.openmct.overlays.progressDialog({
-        progressPerc: 'unknown',
+        progressPerc: null,
         message:
           'Do not navigate away from this page or close this browser tab while this message is displayed.',
         iconClass: 'info',

src/ui/layout/search/SearchResultsDropDown.vue

@@ -50,7 +50,7 @@
         </div>
         <div v-if="searchLoading" class="c-gsearch__result-pane-msg">
           <div class="hint">Searching...</div>
-          <progress-bar :model="{ progressPerc: undefined }" />
+          <progress-bar :model="{ progressPerc: null }" />
         </div>
         <div
           v-if="

src/ui/layout/status-bar/NotificationBanner.vue

@@ -40,7 +40,7 @@
     >
 
     <progress-bar
-      v-if="activeModel.progressPerc !== undefined"
+      v-if="activeModel.progressPerc !== null"
       class="c-message-banner__progress-bar"
       :model="activeModel"
     />
@@ -88,7 +88,7 @@ export default {
     return {
       activeModel: {
         message: undefined,
-        progressPerc: undefined,
+        progressPerc: null,
         progressText: undefined,
         minimized: undefined,
         options: undefined
@@ -178,7 +178,7 @@ export default {
         return;
       }
 
-      if (this.activeModel.progressPerc !== undefined) {
+      if (this.activeModel.progressPerc !== null) {
         maximizedDialog = this.openmct.overlays.progressDialog({
           buttons: [minimizeButton],
           ...this.activeModel