Check if buildopts.get_rpm_whitelist() is really set before using it.

This fixes module builds which sets the `buildopts` for something else
than the RPM whitelist.

Before this commit, when `buildopts` was set, the RPM whitelist was
always taken from the buildopts even if it was not defined there.
This resulted in empty `rpm_whitelist` and therefore no pkglist set
in Koji tag.

In this commit, MBS uses whitelist from `buildopts` only if it is
really set by the module maintainer.

module_build_service/builder/KojiModuleBuilder.py

@@ -531,7 +531,7 @@ class KojiModuleBuilder(GenericBuilder):
             self.tag_name + "-build", self.arches, perm="admin")
 
         buildopts = self.mmd.get_buildopts()
-        if buildopts:
+        if buildopts and buildopts.get_rpm_whitelist():
             rpm_whitelist = buildopts.get_rpm_whitelist()
         else:
             rpm_whitelist = self.components

tests/test_builder/test_koji.py

@@ -487,6 +487,14 @@ class TestKojiBuilder:
             opts.add_rpm_to_whitelist("custom2")
             mmd.set_buildopts(opts)
             self.module.modulemd = mmd_to_str(mmd)
+        else:
+            # Set some irrelevant buildopts options to test that KojiModuleBuilder
+            # is not confused by this.
+            mmd = self.module.mmd()
+            opts = Modulemd.Buildopts()
+            opts.set_rpm_macros("%my_macro 1")
+            mmd.set_buildopts(opts)
+            self.module.modulemd = mmd_to_str(mmd)
 
         if repo_include_all is False:
             mmd = self.module.mmd()