[SPARK-20798] GenerateUnsafeProjection should check if a value is null before calling the getter

## What changes were proposed in this pull request?

GenerateUnsafeProjection.writeStructToBuffer() did not honor the assumption that the caller must make sure that a value is not null before using the getter. This could lead to various errors. This change fixes that behavior.

Example of code generated before:
```scala
/* 059 */         final UTF8String fieldName = value.getUTF8String(0);
/* 060 */         if (value.isNullAt(0)) {
/* 061 */           rowWriter1.setNullAt(0);
/* 062 */         } else {
/* 063 */           rowWriter1.write(0, fieldName);
/* 064 */         }
```

Example of code generated now:
```scala
/* 060 */         boolean isNull1 = value.isNullAt(0);
/* 061 */         UTF8String value1 = isNull1 ? null : value.getUTF8String(0);
/* 062 */         if (isNull1) {
/* 063 */           rowWriter1.setNullAt(0);
/* 064 */         } else {
/* 065 */           rowWriter1.write(0, value1);
/* 066 */         }
```

## How was this patch tested?

Adds GenerateUnsafeProjectionSuite.

Author: Ala Luszczak <ala@databricks.com>

Closes #18030 from ala/fix-generate-unsafe-projection.