Add STRING to allowed first parameter of convert_tz as it returns null for malformatted date

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/expression/datetime/DateTimeFunctions.java

@@ -70,7 +70,6 @@
 import org.opensearch.sql.data.model.ExprValue;
 import org.opensearch.sql.data.type.ExprCoreType;
 import org.opensearch.sql.exception.ExpressionEvaluationException;
-import org.opensearch.sql.exception.SemanticCheckException;
 import org.opensearch.sql.expression.function.BuiltinFunctionName;
 import org.opensearch.sql.expression.function.BuiltinFunctionRepository;
 import org.opensearch.sql.expression.function.DefaultFunctionResolver;
@@ -1282,7 +1281,7 @@ public static ExprValue exprConvertTZ(
     if (startingDateTime.type() == ExprCoreType.STRING) {
       try {
         startingDateTime = new ExprTimestampValue(startingDateTime.stringValue());
-      } catch (SemanticCheckException e) {
+      } catch (ExpressionEvaluationException e) {
         return ExprNullValue.of();
       }
     }

docs/user/ppl/functions/datetime.rst

@@ -118,7 +118,7 @@ Description
 
 Usage: convert_tz(timestamp, from_timezone, to_timezone) constructs a local timestamp converted from the from_timezone to the to_timezone. CONVERT_TZ returns null when any of the three function arguments are invalid, i.e. timestamp is not in the format yyyy-MM-dd HH:mm:ss or the timeszone is not in (+/-)HH:mm. It also is invalid for invalid dates, such as February 30th and invalid timezones, which are ones outside of -13:59 and +14:00.
 
-Argument type: TIMESTAMP, STRING, STRING
+Argument type: TIMESTAMP/STRING, STRING, STRING
 
 Return type: TIMESTAMP