[SPARK-19129][SQL] SessionCatalog: Disallow empty part col values in partition spec

Empty partition column values are not valid for partition specification. Before this PR, we accept users to do it; however, Hive metastore does not detect and disallow it too. Thus, users hit the following strange error.

```Scala
val df = spark.createDataFrame(Seq((0, "a"), (1, "b"))).toDF("partCol1", "name")
df.write.mode("overwrite").partitionBy("partCol1").saveAsTable("partitionedTable")
spark.sql("alter table partitionedTable drop partition(partCol1='')")
spark.table("partitionedTable").show()
```

In the above example, the WHOLE table is DROPPED when users specify a partition spec containing only one partition column with empty values.

When the partition columns contains more than one, Hive metastore APIs simply ignore the columns with empty values and treat it as partial spec. This is also not expected. This does not follow the actual Hive behaviors. This PR is to disallow users to specify such an invalid partition spec in the `SessionCatalog` APIs.

Added test cases

Author: gatorsmile <gatorsmile@gmail.com>

Closes #16583 from gatorsmile/disallowEmptyPartColValue.

(cherry picked from commit a23debd7)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>