[SPARK-15421][SQL] Validate DDL property values

## What changes were proposed in this pull request?

When we parse DDLs involving table or database properties, we need to validate the values.
E.g. if we alter a database's property without providing a value:
```
ALTER DATABASE my_db SET DBPROPERTIES('some_key')
```
Then we'll ignore it with Hive, but override the property with the in-memory catalog. Inconsistencies like these arise because we don't validate the property values.

In such cases, we should throw exceptions instead.

## How was this patch tested?

`DDLCommandSuite`

Author: Andrew Or <andrew@databricks.com>

Closes #13205 from andrewor14/ddl-prop-values.