leave a (temporary) warning if there are no KeyStoreSources

This is expected to be a temporary measure pending JENKINS-73335
as such no tests are introduced for this, rather manual testing was
performed.

src/main/resources/com/cloudbees/plugins/credentials/impl/CertificateCredentialsImpl/credentials.jelly

@@ -26,7 +26,15 @@
 
 <j:jelly xmlns:j="jelly:core" xmlns:f="/lib/form" xmlns:st="jelly:stapler">
   <f:entry title="${%Certificate}" field="keyStoreSource">
-    <f:hetero-radio field="keyStoreSource" descriptors="${descriptor.getPropertyType('keyStoreSource').applicableDescriptors}"/>
+    <j:set var="keyStoreSourceDescriptors" value="${descriptor.getPropertyType('keyStoreSource').applicableDescriptors}"/>
+    <j:choose>
+        <j:when test="${keyStoreSourceDescriptors.isEmpty()}">
+            <div class="warning">Jenkins has no <code>KeyStoreSources</code> available, Certificate credentials will not be able to be created.</div>
+        </j:when>
+        <j:otherwise>
+            <f:hetero-radio field="keyStoreSource" descriptors="${keyStoreSourceDescriptors}"/>
+        </j:otherwise>
+    </j:choose>
   </f:entry>
   <f:entry title="${%Password}" field="password">
     <f:password value="${instance==null || instance.passwordEmpty ? '' : instance.password}"/>