chore: remove unnecessary null checks in template variable validation

- Simplified code by dropping redundant null checks

Signed-off-by: csbiy <[email protected]>

Author: csbiy

spring-ai-template-st/src/main/java/org/springframework/ai/template/st/StTemplateRenderer.java

@@ -128,7 +128,7 @@ private ST createST(String template) {
 	 */
 	private Set<String> validate(ST st, Map<String, Object> templateVariables) {
 		Set<String> templateTokens = getInputVariables(st);
-		Set<String> modelKeys = templateVariables != null ? templateVariables.keySet() : new HashSet<>();
+		Set<String> modelKeys = templateVariables.keySet();
 		Set<String> missingVariables = new HashSet<>(templateTokens);
 		missingVariables.removeAll(modelKeys);