[SPARK-15462][SQL][TEST] unresolved === false` is enough in testcases.

## What changes were proposed in this pull request?

In only `catalyst` module, there exists 8 evaluation test cases on unresolved expressions. But, in real-world situation, those cases doesn't happen since they occurs exceptions before evaluations.
```scala
scala> sql("select format_number(null, 3)")
res0: org.apache.spark.sql.DataFrame = [format_number(CAST(NULL AS DOUBLE), 3): string]
scala> sql("select format_number(cast(null as NULL), 3)")
org.apache.spark.sql.catalyst.parser.ParseException:
DataType null() is not supported.(line 1, pos 34)
```

This PR makes those testcases more realistic.
```scala
-    checkEvaluation(FormatNumber(Literal.create(null, NullType), Literal(3)), null)
+    assert(FormatNumber(Literal.create(null, NullType), Literal(3)).resolved === false)
```
Also, this PR also removes redundant `resolved` checking in `FoldablePropagation` optimizer.

## How was this patch tested?

Pass the modified Jenkins tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13241 from dongjoon-hyun/SPARK-15462.