refactor: Update the defaults for our .env files (#459)

Nulls can provide undesired behavior so we want to switch to empty strings

.env

@@ -1,33 +1,33 @@
 NODE_ENV='production'
-ACCESS_TOKEN_COOKIE_NAME=null
-BASE_URL=null
-CREDENTIALS_BASE_URL=null
-CSRF_TOKEN_API_PATH=null
-DISCOVERY_API_BASE_URL=null
-ENTERPRISE_LEARNER_PORTAL_HOSTNAME=null
-ECOMMERCE_BASE_URL=null
+ACCESS_TOKEN_COOKIE_NAME=''
+BASE_URL=''
+CREDENTIALS_BASE_URL=''
+CSRF_TOKEN_API_PATH=''
+DISCOVERY_API_BASE_URL=''
+ECOMMERCE_BASE_URL=''
+ENTERPRISE_LEARNER_PORTAL_HOSTNAME=''
 IGNORED_ERROR_REGEX=''
-INSIGHTS_BASE_URL=null
-LANGUAGE_PREFERENCE_COOKIE_NAME=null
-LMS_BASE_URL=null
-LOGIN_URL=null
-LOGOUT_URL=null
-LOGO_URL=null
-LOGO_TRADEMARK_URL=null
-LOGO_WHITE_URL=null
-FAVICON_URL=null
-MARKETING_SITE_BASE_URL=null
-ORDER_HISTORY_URL=null
-REFRESH_ACCESS_TOKEN_ENDPOINT=null
-SEARCH_CATALOG_URL=null
-SEGMENT_KEY=null
-SITE_NAME=null
-SOCIAL_UTM_MILESTONE_CAMPAIGN=null
-STUDIO_BASE_URL=null
-SUPPORT_URL=null
-SUPPORT_URL_CALCULATOR_MATH=null
-SUPPORT_URL_ID_VERIFICATION=null
-SUPPORT_URL_VERIFIED_CERTIFICATE=null
-TWITTER_HASHTAG=null
-TWITTER_URL=null
-USER_INFO_COOKIE_NAME=null
+INSIGHTS_BASE_URL=''
+LANGUAGE_PREFERENCE_COOKIE_NAME=''
+LMS_BASE_URL=''
+LOGIN_URL=''
+LOGOUT_URL=''
+LOGO_URL=''
+LOGO_TRADEMARK_URL=''
+LOGO_WHITE_URL=''
+FAVICON_URL=''
+MARKETING_SITE_BASE_URL=''
+ORDER_HISTORY_URL=''
+REFRESH_ACCESS_TOKEN_ENDPOINT=''
+SEARCH_CATALOG_URL=''
+SEGMENT_KEY=''
+SITE_NAME=''
+SOCIAL_UTM_MILESTONE_CAMPAIGN=''
+STUDIO_BASE_URL=''
+SUPPORT_URL=''
+SUPPORT_URL_CALCULATOR_MATH=''
+SUPPORT_URL_ID_VERIFICATION=''
+SUPPORT_URL_VERIFIED_CERTIFICATE=''
+TWITTER_HASHTAG=''
+TWITTER_URL=''
+USER_INFO_COOKIE_NAME=''

.env.development

@@ -20,7 +20,7 @@ ORDER_HISTORY_URL='http://localhost:1996/orders'
 PORT=2000
 REFRESH_ACCESS_TOKEN_ENDPOINT='http://localhost:18000/login_refresh'
 SEARCH_CATALOG_URL='http://localhost:18000/courses'
-SEGMENT_KEY=null
+SEGMENT_KEY=''
 SITE_NAME='edX'
 SOCIAL_UTM_MILESTONE_CAMPAIGN='edxmilestone'
 STUDIO_BASE_URL='http://localhost:18010'

.env.test

@@ -20,7 +20,7 @@ ORDER_HISTORY_URL='http://localhost:1996/orders'
 PORT=2000
 REFRESH_ACCESS_TOKEN_ENDPOINT='http://localhost:18000/login_refresh'
 SEARCH_CATALOG_URL='http://localhost:18000/courses'
-SEGMENT_KEY=null
+SEGMENT_KEY=''
 SITE_NAME='edX'
 SOCIAL_UTM_MILESTONE_CAMPAIGN='edxmilestone'
 STUDIO_BASE_URL='http://localhost:18010'