[feat](api) add nullable info for api _schema (apache#46557)

### What problem does this PR solve?
spark doris connector still use `/api/<db>/<table>/_schema` api to get
column info, here we need nullable information.

like information_schema.columns, we add property: `is_nullable = Yes or
No`

fe/fe-core/src/main/java/org/apache/doris/httpv2/rest/TableSchemaAction.java

@@ -109,6 +109,7 @@ protected Object schema(
                         Optional aggregationType = Optional.ofNullable(column.getAggregationType());
                         baseInfo.put("aggregation_type", aggregationType.isPresent()
                                 ? column.getAggregationType().toSql() : "");
+                        baseInfo.put("is_nullable", column.isAllowNull() ? "Yes" : "No");
                         propList.add(baseInfo);
                     }
                     resultMap.put("status", 200);