Address PR review feedback

Check for key existence, not key truthiness.

Author: jportner

src/core/server/elasticsearch/elasticsearch_config.test.ts

@@ -331,7 +331,7 @@ describe('deprecations', () => {
   });
 
   it('logs a warning if ssl.key is set and ssl.certificate is not', () => {
-    const { messages } = applyElasticsearchDeprecations({ ssl: { key: 'foo' } });
+    const { messages } = applyElasticsearchDeprecations({ ssl: { key: '' } });
     expect(messages).toMatchInlineSnapshot(`
       Array [
         "Setting [${CONFIG_PATH}.ssl.key] without [${CONFIG_PATH}.ssl.certificate] is deprecated. This has no effect, you should use both settings to enable TLS client authentication to Elasticsearch.",
@@ -340,13 +340,18 @@ describe('deprecations', () => {
   });
 
   it('logs a warning if ssl.certificate is set and ssl.key is not', () => {
-    const { messages } = applyElasticsearchDeprecations({ ssl: { certificate: 'foo' } });
+    const { messages } = applyElasticsearchDeprecations({ ssl: { certificate: '' } });
     expect(messages).toMatchInlineSnapshot(`
       Array [
         "Setting [${CONFIG_PATH}.ssl.certificate] without [${CONFIG_PATH}.ssl.key] is deprecated. This has no effect, you should use both settings to enable TLS client authentication to Elasticsearch.",
       ]
     `);
   });
+
+  it('does not log a warning if both ssl.key and ssl.certificate are set', () => {
+    const { messages } = applyElasticsearchDeprecations({ ssl: { key: '', certificate: '' } });
+    expect(messages).toEqual([]);
+  });
 });
 
 test('#username throws if equal to "elastic", only while running from source', () => {

src/core/server/elasticsearch/elasticsearch_config.ts

@@ -117,11 +117,11 @@ const deprecations: ConfigDeprecationProvider = () => [
         `Setting [${fromPath}.username] to "elastic" is deprecated. You should use the "kibana" user instead.`
       );
     }
-    if (es.ssl?.key && !es.ssl?.certificate) {
+    if (es.ssl?.key !== undefined && es.ssl?.certificate === undefined) {
       log(
         `Setting [${fromPath}.ssl.key] without [${fromPath}.ssl.certificate] is deprecated. This has no effect, you should use both settings to enable TLS client authentication to Elasticsearch.`
       );
-    } else if (es.ssl?.certificate && !es.ssl?.key) {
+    } else if (es.ssl?.certificate !== undefined && es.ssl?.key === undefined) {
       log(
         `Setting [${fromPath}.ssl.certificate] without [${fromPath}.ssl.key] is deprecated. This has no effect, you should use both settings to enable TLS client authentication to Elasticsearch.`
       );