Handle empty results and non-multiple sarifs better

src/main/kotlin/slack/cli/sarif/MergeSarifReports.kt

@@ -290,9 +290,14 @@ public class MergeSarifReports : CliktCommand(help = DESCRIPTION) {
 
   private fun merge(inputs: List<Path>) {
     log("Parsing ${inputs.size} sarif files")
+    val parsed = loadSarifs(inputs)
     val mergedSarif =
-      loadSarifs(inputs)
-        .merge(levelOverride = level, removeUriPrefixes = removeUriPrefixes, log = ::log)
+      when (parsed.size) {
+        0 -> error("No sarif files parsed. Consider using --allow-empty")
+        1 -> parsed[0]
+        else ->
+          parsed.merge(levelOverride = level, removeUriPrefixes = removeUriPrefixes, log = ::log)
+      }
     log("Writing merged sarif to $outputFile")
     prepareOutput()
     outputFile.writeText(SarifSerializer.toJson(mergedSarif))

src/main/kotlin/slack/cli/sarif/SarifUtil.kt

@@ -116,9 +116,9 @@ internal fun List<SarifSchema210>.merge(
   removeUriPrefixes: Boolean = false,
   log: (String) -> Unit,
 ): SarifSchema210 {
+  check(isNotEmpty()) { "Must have at least one sarif file to merge!" }
+
   log("Merging $size sarif files")
-  val sortedMergedRules =
-    flatMap { it.runs.single().tool.driver.rules.orEmpty() }.associateBy { it.id }.toSortedMap()
   val mergedResults =
     flatMap { it.runs.single().results.orEmpty() }
       // Some projects produce multiple reports for different variants, so we need to
@@ -127,6 +127,14 @@ internal fun List<SarifSchema210>.merge(
       .distinctBy { it.shallowHash }
       .also { log("Merged ${it.size} results") }
 
+  if (mergedResults.isEmpty()) {
+    // Nothing to do here, just return the first
+    return this[0]
+  }
+
+  val sortedMergedRules =
+    flatMap { it.runs.single().tool.driver.rules.orEmpty() }.associateBy { it.id }.toSortedMap()
+
   // Update rule.ruleIndex to match the index in rulesToAdd
   val ruleIndicesById =
     sortedMergedRules.entries.withIndex().associate { (index, entry) -> entry.key to index }