infoschema: fix pseudo data for profiling table

[alivxxx]

What problem does this PR solve?

When selecting rows from profiling, tidb will get panics like:

encoding/binary.binary.littleEndian.PutUint64(...)
/usr/local/go/src/encoding/binary/binary.go:82
github.com/pingcap/tidb/util/chunk.MutRow.SetDatum(0xc4205a9880, 0x0, 0xf, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
/home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/util/chunk/mutrow.go:283 +0x8a1
github.com/pingcap/tidb/util/chunk.MutRow.SetDatums(0xc4205a9880, 0x0, 0xc42017d400, 0x12, 0x12)
/home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/util/chunk/mutrow.go:270 +0xe9
github.com/pingcap/tidb/executor.(*TableScanExec).nextChunk4InfoSchema.func1(0x0, 0xc42017d400, 0x12, 0x12, 0xc42254ab40, 0x12, 0x12, 0x1, 0x1, 0x0)
/home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/executor/executor.go:786 +0x68
github.com/pingcap/tidb/infoschema.(*infoschemaTable).IterRecords(0xc420627940, 0x142e980, 0xc420e74e00, 0x0, 0x0, 0x0, 0xc42254ab40, 0x12, 0x12, 0xc4205a98c0, ...)
/home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/infoschema/tables.go:1169 +0x123
github.com/pingcap/tidb/executor.(*TableScanExec).nextChunk4InfoSchema(0xc4212d5860, 0x7f92db394118, 0xc42159f000, 0xc4205a95e0, 0x8b53b9, 0xc4205a95e0)
/home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/executor/executor.go:785 +0x41d

What is changed and how it works?

The type of SOURCE_FUNCTION and SOURCE_FILE is varchar, so the default value should be an empty string, not 0.

Check List

Tests

• Unit test

Code changes

• Has unexported function/method change

Side effects

• None

Related changes

• Need to cherry-pick to the release branch

PTAL @zz-jason @winoros @eurekaka

[winoros]

lgtm

[XuHuaiyu]

LGTM

[XuHuaiyu]

/run-all-tests