fix: added null condition check (closes #13)

Author: niftylettuce

src/index.js

@@ -13,14 +13,15 @@ export default (env, options) => {
 
   Object.keys(env).forEach(key => {
     debug(`key "${key}" before type was ${typeof env[key]}`);
-    env[key] = parseKey(env[key], key);
-    debug(`key "${key}" after type was ${typeof env[key]}`);
-
-    if (envOptions.assignToProcessEnv === true) {
-      if (envOptions.overrideProcessEnv === true) {
-        process.env[key] = env[key] || process.env[key];
-      } else {
-        process.env[key] = process.env[key] || env[key];
+    if (env[key]) {
+      env[key] = parseKey(env[key], key);
+      debug(`key "${key}" after type was ${typeof env[key]}`);
+      if (envOptions.assignToProcessEnv === true) {
+        if (envOptions.overrideProcessEnv === true) {
+          process.env[key] = env[key] || process.env[key];
+        } else {
+          process.env[key] = process.env[key] || env[key];
+        }
       }
     }
   });