Do not attempt to generate prebuilt_kernel_modules if they are not src

Some configurations do not have checked-in prebuilt .ko files.

Test: go build ./fsgen
Bug: 379478434
Change-Id: Icd0febf2ccc59655a06b7382ffd102652bc6f2bb
diff --git a/fsgen/filesystem_creator.go b/fsgen/filesystem_creator.go
index c74579a..dca44f0 100644
--- a/fsgen/filesystem_creator.go
+++ b/fsgen/filesystem_creator.go
@@ -320,7 +320,7 @@
 	}
 	switch partitionType {
 	case "system_dlkm":
-		props.Srcs = ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.SystemKernelModules
+		props.Srcs = android.ExistentPathsForSources(ctx, ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.SystemKernelModules).Strings()
 		props.System_dlkm_specific = proptools.BoolPtr(true)
 		if len(ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.SystemKernelLoadModules) == 0 {
 			// Create empty modules.load file for system
@@ -331,7 +331,7 @@
 			props.Blocklist_file = proptools.StringPtr(blocklistFile)
 		}
 	case "vendor_dlkm":
-		props.Srcs = ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.VendorKernelModules
+		props.Srcs = android.ExistentPathsForSources(ctx, ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.VendorKernelModules).Strings()
 		if len(ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.SystemKernelModules) > 0 {
 			props.System_deps = []string{":" + generatedModuleName(ctx.Config(), "system_dlkm-kernel-modules") + "{.modules}"}
 		}
@@ -340,7 +340,7 @@
 			props.Blocklist_file = proptools.StringPtr(blocklistFile)
 		}
 	case "odm_dlkm":
-		props.Srcs = ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.OdmKernelModules
+		props.Srcs = android.ExistentPathsForSources(ctx, ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.OdmKernelModules).Strings()
 		props.Odm_dlkm_specific = proptools.BoolPtr(true)
 		if blocklistFile := ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.OdmKernelBlocklistFile; blocklistFile != "" {
 			props.Blocklist_file = proptools.StringPtr(blocklistFile)