refactor(接口测试): 变量断言正则匹配，优化变量不存在报错提示

--bug=1039695 --user=陈建星 【接口测试】定义-断言-值为josn和list断言失败 https://www.tapd.cn/55049933/s/1503406

backend/services/api-test/src/main/java/io/metersphere/api/parser/jmeter/processor/assertion/VariableAssertionConverter.java

@@ -81,7 +81,11 @@ public class VariableAssertionConverter extends AssertionConverter<MsVariableAss
         handleMap.put(MsAssertionCondition.REGEX.name(),
                 """
                 import java.util.regex.Pattern;
-                result = Pattern.matches(Pattern.quote(expectation), variableValue);
+                if (variableValue != null) {
+                   result = Pattern.matches(Pattern.quote(expectation), variableValue);
+                } else {
+                   result = false;
+                }
                 msg = variableValue + " not matching " + expectation;
                 """);