Fix droiddoc disting when WITHOUT_CHECK_API is true.

When WITHOUT_CHECK_API=true, the droidstubs apiFile can be nil. Fix this
by defensively checking for nil paths when making the TaggedDistFiles,
and also in android.MakeDefaultDistFiles.

The error message reported in r.android.com/1335521 was:

"internal error: panic in GenerateBuildActions for singleton androidmk
  Dist file should not be nil for the default tag in
  android.net.ipsec.ike.stubs.source in translateAndroidMkModule for
  module android.net.ipsec.ike.stubs.source variant android_common"

Test: WITHOUT_CHECK_API=true m droid dist

Bug: 152834186
Signed-off-by: Jingwen Chen <jingwen@google.com>
Change-Id: I1b1f7c0b7a0e1c0ed5e15957d0162c47fd3ec197

android/androidmk.go

@@ -163,12 +163,15 @@ func (a *AndroidMkEntries) GetDistForGoals(mod blueprint.Module) []string {
 	name := amod.BaseModuleName()
 
 	var ret []string
+	var availableTaggedDists TaggedDistFiles
 
-	availableTaggedDists := TaggedDistFiles{}
 	if a.DistFiles != nil {
 		availableTaggedDists = a.DistFiles
 	} else if a.OutputFile.Valid() {
 		availableTaggedDists = MakeDefaultDistFiles(a.OutputFile.Path())
+	} else {
+		// Nothing dist-able for this module.
+		return nil
 	}
 
 	// Iterate over this module's dist structs, merged from the dist and dists properties.

android/module.go

@@ -565,6 +565,12 @@ type commonProperties struct {
 type TaggedDistFiles map[string]Paths
 
 func MakeDefaultDistFiles(paths ...Path) TaggedDistFiles {
+	for _, path := range paths {
+		if path == nil {
+			panic("The path to a dist file cannot be nil.")
+		}
+	}
+
 	// The default OutputFile tag is the empty "" string.
 	return TaggedDistFiles{"": paths}
 }

java/androidmk.go

@@ -563,15 +563,21 @@ func (dstubs *Droidstubs) AndroidMkEntries() []android.AndroidMkEntries {
 	// are created in make if only the api txt file is being generated. This is
 	// needed because an invalid output file would prevent the make entries from
 	// being written.
+	//
+	// Note that dstubs.apiFile can be also be nil if WITHOUT_CHECKS_API is true.
 	// TODO(b/146727827): Revert when we do not need to generate stubs and API separately.
-	distFile := dstubs.apiFile
+
+	var distFiles android.TaggedDistFiles
+	if dstubs.apiFile != nil {
+		distFiles = android.MakeDefaultDistFiles(dstubs.apiFile)
+	}
 	outputFile := android.OptionalPathForPath(dstubs.stubsSrcJar)
 	if !outputFile.Valid() {
-		outputFile = android.OptionalPathForPath(distFile)
+		outputFile = android.OptionalPathForPath(dstubs.apiFile)
 	}
 	return []android.AndroidMkEntries{android.AndroidMkEntries{
 		Class:      "JAVA_LIBRARIES",
-		DistFiles:  android.MakeDefaultDistFiles(distFile),
+		DistFiles:  distFiles,
 		OutputFile: outputFile,
 		Include:    "$(BUILD_SYSTEM)/soong_droiddoc_prebuilt.mk",
 		ExtraEntries: []android.AndroidMkExtraEntriesFunc{