Fixing issues with the url.search being null in Node 10 (#26992)

Node 10 uses `null` to denote the non-existence of a querystring
parameter when `url.parse(urlString, true)` is used, the following
changes fix our usages within the security plugin.

x-pack/plugins/security/server/lib/authentication/providers/basic.js

@@ -86,7 +86,7 @@ export class BasicAuthenticationProvider {
     // Query string may contain the path where logout has been called or
     // logout reason that login page may need to know.
     return DeauthenticationResult.redirectTo(
-      `${this._options.basePath}/login${request.url.search}`
+      `${this._options.basePath}/login${request.url.search || ''}`
     );
   }
 

x-pack/plugins/security/server/lib/authentication/providers/saml.js

@@ -403,7 +403,7 @@ export class SAMLAuthenticationProvider {
       logoutArgs = [
         'shield.samlInvalidate',
         // Elasticsearch expects `queryString` without leading `?`, so we should strip it with `slice`.
-        { body: { queryString: request.url.search.slice(1), acs: this._getACS() } }
+        { body: { queryString: request.url.search ? request.url.search.slice(1) : '', acs: this._getACS() } }
       ];
     } else {
       this._options.log(['debug', 'security', 'saml'], 'Logout has been initiated by the user.');