bp2build: support strip properties.

This CL adds support to bp2build for generating all strip-related
properties into a strip dictionary as a parameter in the cc_library
macro.

With the dictionary, it's easy to organize related attributes and
directly expand into a stripped_shared_library's attributes.

Test: //build/bazel/tests/bionic:compare_libc_stripping
Test: TH
Fixes: b/187928597
Change-Id: Ifea68d48fe295e71a43b12876cb168c475a62187
diff --git a/bp2build/build_conversion.go b/bp2build/build_conversion.go
index 4de5aae..a1e0424 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -488,6 +488,9 @@
 		ret = "{\n"
 		// Sort and print the struct props by the key.
 		structProps := extractStructProperties(propertyValue, indent)
+		if len(structProps) == 0 {
+			return "", nil
+		}
 		for _, k := range android.SortedStringKeys(structProps) {
 			ret += makeIndent(indent + 1)
 			ret += fmt.Sprintf("%q: %s,\n", k, structProps[k])