don't try to parse empty install_requires

lib/fpm/package/pyfpm/get_metadata.py

@@ -48,18 +48,18 @@ def run(self):
     # end if
 
     final_deps = []
-    for dep in pkg_resources.parse_requirements(self.distribution.install_requires):
-        # add all defined specs to the dependecy list separately.
-        if dep.specs:
-            for operator, version in dep.specs:
-                final_deps.append("%s %s %s" % (
-                    dep.project_name,
-                    "=" if operator == "==" else operator,
-                    version
-                ))
-        else:
-            final_deps.append(dep.project_name)            
-
+    if self.distribution.install_requires:
+        for dep in pkg_resources.parse_requirements(self.distribution.install_requires):
+            # add all defined specs to the dependecy list separately.
+            if dep.specs:
+                for operator, version in dep.specs:
+                    final_deps.append("%s %s %s" % (
+                        dep.project_name,
+                        "=" if operator == "==" else operator,
+                        version
+                    ))
+            else:
+                final_deps.append(dep.project_name)            
 
     data["dependencies"] = final_deps