fix(masthead-search): define placeholder (#3997)

### Related Ticket(s)

Refs #3884.

### Changelog

**New**

- Default placeholder in `<dds-masthead-search>`.

packages/web-components/src/components/masthead/masthead-composite.ts

@@ -228,6 +228,12 @@ class DDSMastheadComposite extends LitElement {
   @property({ type: Boolean, reflect: true, attribute: 'open-search-dropdown' })
   openSearchDropdown = false;
 
+  /**
+   * Value to display when the input has an empty `value`.
+   */
+  @property()
+  searchPlaceholder?: string;
+
   /**
    * The user authentication status.
    */
@@ -271,6 +277,7 @@ class DDSMastheadComposite extends LitElement {
       menuButtonAssistiveTextInactive,
       language,
       openSearchDropdown,
+      searchPlaceholder,
       unauthenticatedProfileItems,
       userStatus,
       _loadSearchResults: loadSearchResults,
@@ -298,6 +305,7 @@ class DDSMastheadComposite extends LitElement {
           input-timeout="${inputTimeout}"
           language="${ifNonNull(language)}"
           ?open="${openSearchDropdown}"
+          placeholder="${ifNonNull(searchPlaceholder)}"
           .currentSearchResults="${ifNonNull(currentSearchResults)}"
           ._loadSearchResults="${ifNonNull(loadSearchResults)}"
         ></dds-masthead-search-composite>

packages/web-components/src/components/masthead/masthead-search-composite.ts

@@ -9,6 +9,7 @@
 
 import { html, property, customElement, LitElement } from 'lit-element';
 import ddsSettings from '@carbon/ibmdotcom-utilities/es/utilities/settings/settings.js';
+import ifNonNull from 'carbon-web-components/es/globals/directives/if-non-null.js';
 import HybridRenderMixin from '../../globals/mixins/hybrid-render';
 import ThrottedInputMixin from '../../globals/mixins/throttled-input';
 import './masthead-search';
@@ -81,6 +82,12 @@ class DDSMastheadSearchComposite extends ThrottedInputMixin(HybridRenderMixin(Li
   @property({ type: Boolean, reflect: true })
   open = false;
 
+  /**
+   * Value to display when the input has an empty `value`.
+   */
+  @property()
+  placeholder?: string;
+
   /**
    * The shadow slot this search UI should be in.
    */
@@ -104,9 +111,9 @@ class DDSMastheadSearchComposite extends ThrottedInputMixin(HybridRenderMixin(Li
   }
 
   renderLightDOM() {
-    const { active, currentSearchResults, open } = this;
+    const { active, currentSearchResults, open, placeholder } = this;
     return html`
-      <dds-masthead-search ?active="${active}" ?open="${open}">
+      <dds-masthead-search ?active="${active}" ?open="${open}" placeholder="${ifNonNull(placeholder)}">
         ${currentSearchResults.map(
           item =>
             html`

packages/web-components/src/components/masthead/masthead-search.ts

@@ -341,7 +341,7 @@ class DDSMastheadSearch extends BXDropdown {
    * Value to display when the input has an empty `value`.
    */
   @property({ reflect: true })
-  placeholder = '';
+  placeholder = 'Search all of IBM';
 
   /**
    * The redirect URL when a user selects a search suggestion.