Propagate empty vs unspecified system_shared_libs correctly.

Necessary to get correct prebuilts for many Bionic libs.

Cleaned up numerious "system_shared_libs: []" from test fixtures, since
they otherwise would need correction in the expected results, and it is
better to have a single test focused on testing system_shared_libs
propagation.

Test: m nothing
Bug: 152255951
Change-Id: If2e8a5296223e6281d833312660e8e9e4cd184c0
diff --git a/cc/library_sdk_member.go b/cc/library_sdk_member.go
index 4967455..0a11af1 100644
--- a/cc/library_sdk_member.go
+++ b/cc/library_sdk_member.go
@@ -212,7 +212,9 @@
 		outputProperties.AddPropertyWithTag("shared_libs", libInfo.SharedLibs, builder.SdkMemberReferencePropertyTag(false))
 	}
 
-	if len(libInfo.SystemSharedLibs) > 0 {
+	// SystemSharedLibs needs to be propagated if it's a list, even if it's empty,
+	// so check for non-nil instead of nonzero length.
+	if libInfo.SystemSharedLibs != nil {
 		outputProperties.AddPropertyWithTag("system_shared_libs", libInfo.SystemSharedLibs, builder.SdkMemberReferencePropertyTag(false))
 	}
 
@@ -327,7 +329,8 @@
 	// This field is exported as its contents may not be arch specific.
 	SharedLibs []string
 
-	// The set of system shared libraries
+	// The set of system shared libraries. Note nil and [] are semantically
+	// distinct - see BaseLinkerProperties.System_shared_libs.
 	//
 	// This field is exported as its contents may not be arch specific.
 	SystemSharedLibs []string