Fix a bug that occur when plugin pkg-config requirements are empty (#9238)

Summary:
Fix a bug introduced by https://github.com/facebook/rocksdb/issues/9198. The bug is triggered when a plugin does not provide any pkg-config requirements.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9238

Reviewed By: riversand963

Differential Revision: D32771406

Pulled By: ajkr

fbshipit-source-id: 79301871a8bf4e624d5e5eb9d219d7f13948c64d

Makefile

@@ -253,7 +253,7 @@ ROCKSDB_PLUGIN_PKGCONFIG_REQUIRES = $(foreach plugin, $(ROCKSDB_PLUGINS), $($(pl
 PLATFORM_LDFLAGS += $(foreach plugin, $(ROCKSDB_PLUGINS), $($(plugin)_LDFLAGS))
 CXXFLAGS += $(foreach plugin, $(ROCKSDB_PLUGINS), $($(plugin)_CXXFLAGS))
 
-ifneq ($(ROCKSDB_PLUGIN_PKGCONFIG_REQUIRES),)
+ifneq ($(strip $(ROCKSDB_PLUGIN_PKGCONFIG_REQUIRES)),)
 LDFLAGS := $(LDFLAGS) $(shell pkg-config --libs $(ROCKSDB_PLUGIN_PKGCONFIG_REQUIRES))
 ifneq ($(.SHELLSTATUS),0)
 $(error pkg-config failed)