toMatchDisk throws StringIndexOutOfBoundsException when serializing an empty snapshot

[pandaman64]

Summary

I encountered an issue in the Selfie library when comparing snapshots stored on disk. When I add a new facet to a snapshot that was already stored on disk, the comparison failed with a StringIndexOutOfBoundsException instead of showing a pretty diff message.

The exception occurs during the pretty-printing of the comparison results. It seems that the expected facet didn't have any content to print when adding a new facet. So, when the code attempts to remove the last character from the string buffer, it results in the out-of-bounds exception.

Reproduction

Step 1: Create a test with a snapshot

val snapshot = Snapshot.of("")
expectSelfie(snapshot).toMatchDisk_TODO("reproduce_selfie_issue")
```kotlin

### Step 2: Run the test again with a facet added to the snapshot

```kotlin
val snapshot = Snapshot
    .of("")
    .plusFacet("new-facet", "new-facet-value")
expectSelfie(snapshot).toMatchDisk("reproduce_selfie_issue")

Expected result:
A nice diff showing the facet mismatch between snapshots.

Actual result:

I got an exception with the following stack trace:

String index out of range: -1
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
	at java.base/java.lang.AbstractStringBuilder.setLength(AbstractStringBuilder.java:319)
	at java.base/java.lang.StringBuilder.setLength(StringBuilder.java:91)
	at com.diffplug.selfie.SelfieImplementationsKt.serializeOnlyFacets(SelfieImplementations.kt:229)
	at com.diffplug.selfie.SelfieImplementationsKt.assertEqual(SelfieImplementations.kt:270)
	at com.diffplug.selfie.SelfieImplementationsKt.access$assertEqual(SelfieImplementations.kt:1)
	at com.diffplug.selfie.DiskSelfie.toMatchDisk(SelfieImplementations.kt:41)
	at com.diffplug.selfie.StringSelfie.toMatchDisk(SelfieImplementations.kt:177)

Details

The exception occurs at SelfieImplementations.kt line 229:

Since there is no diff from the existing snapshot on the disk (only the actual snapshot has a diff), the writer StringBuilder remains empty during the serialization process. Attempting to remove the last character with writer.setLength(writer.length - 1) causes the StringIndexOutOfBoundsException since writer.length is 0.

Thank you for the great library!