do not allocate empty strings

follow review from @pwoolcoc, and do not use

    SafeString::new(&<String>::new())

since this makes an allocation which will then just be thrown away.
Instead, we pass ""

plume-models/src/blogs.rs

@@ -143,8 +143,8 @@ impl Blog {
                     name: inst.clone(),
                     local: false,
                     // We don't really care about all the following for remote instances
-                    long_description: SafeString::new(&<String>::new()),
-                    short_description: SafeString::new(&<String>::new()),
+                    long_description: SafeString::new(""),
+                    short_description: SafeString::new(""),
                     default_license: String::new(),
                     open_registrations: true,
                     short_description_html: String::new(),

plume-models/src/users.rs

@@ -205,8 +205,8 @@ impl User {
                     public_domain: inst.clone(),
                     local: false,
                     // We don't really care about all the following for remote instances
-                    long_description: SafeString::new(&<String>::new()),
-                    short_description: SafeString::new(&<String>::new()),
+                    long_description: SafeString::new(""),
+                    short_description: SafeString::new(""),
                     default_license: String::new(),
                     open_registrations: true,
                     short_description_html: String::new(),