fix handling of nothing in init_load_path (#54382)

As-written, this was checking for `nothing` after the `String` convert,
which will obviously throw a MethodError.

---------

Co-authored-by: Jeff Bezanson <jeff.bezanson@gmail.com>
Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>

base/initdefs.jl

@@ -245,8 +245,14 @@ function init_load_path()
     if haskey(ENV, "JULIA_LOAD_PATH")
         paths = parse_load_path(ENV["JULIA_LOAD_PATH"])
     else
-        paths = filter!(env -> env !== nothing,
-            String[env == "@." ? current_project() : env for env in DEFAULT_LOAD_PATH])
+        paths = String[]
+        for env in DEFAULT_LOAD_PATH
+            if env == "@."
+                env = current_project()
+                env === nothing && continue
+            end
+            push!(paths, env)
+        end
     end
     append!(empty!(LOAD_PATH), paths)
 end