Added guard to asset helper for missing paths

refs #10496

- currently {{asset this/is/not/a.string}} would throw a 500 error
- this commit changes that to make it throw a sensible 400 + incorrect usage error

core/server/helpers/asset.js

@@ -4,12 +4,20 @@
 // Returns the path to the specified asset. The ghost flag outputs the asset path for the Ghost admin
 const proxy = require('./proxy'),
     get = require('lodash/get'),
+    i18n = proxy.i18n,
+    errors = proxy.errors,
     getAssetUrl = proxy.metaData.getAssetUrl,
     SafeString = proxy.SafeString;
 
 module.exports = function asset(path, options) {
     const hasMinFile = get(options, 'hash.hasMinFile');
 
+    if (!path) {
+        throw new errors.IncorrectUsageError({
+            message: i18n.t('warnings.helpers.asset.pathIsRequired')
+        });
+    }
+
     return new SafeString(
         getAssetUrl(path, hasMinFile)
     );

core/server/translations/en.json

@@ -519,6 +519,9 @@
             "flagMustBeEnabled": "The {flagName} flag must be enabled in labs if you wish to use the \\{\\{{helperName}\\}\\} helper.",
             "seeLink": "See {url}",
             "mustBeCalledAsBlock": "The \\{\\{{helperName}\\}\\} helper must be called as a block. E.g.  \\{\\{#{helperName}\\}\\} \\{\\{/{helperName}\\}\\}",
+            "asset": {
+                "pathIsRequired": "The \\{\\{asset\\}\\} helper must be passed a path"
+            },
             "foreach": {
                 "iteratorNeeded": "Need to pass an iterator to #foreach"
             },