ignore empty module rather than regarding it as { default: undefined }

BREAKING CHANGE: Empty modules are now simply ignored.

packages/vite-plugin-minissg/README.md

@@ -392,7 +392,7 @@ Intuitively:
 
 1. A module may have `main` function that returns another module.
    This allows the module to delegate file generation to another
-   module.
+   module, which is possibly dynamically created.
 2. A module may also have `default` value that gives Minissg the
    content of a destination file.
    The content can be given in several forms including string,
@@ -414,7 +414,7 @@ Intuitively:
    See [Static Routing](#static-routing) section below for the details
    of name concatination.
 
-Empty module is regarded as `{ default: undefined }`.
+Empty modules are simply ignored.
 
 Through the `main` functions and mapping objects, the collection of
 modules constitute a tree structure.

packages/vite-plugin-minissg/src/module.ts

@@ -115,8 +115,7 @@ export const run = async (
         const context = { ...con, moduleName, module, path, parent: con }
         return { context: Object.freeze(context), loaded: newLoaded }
       })
-      const subtrees = (await Promise.all(children)).filter(isNotNull)
-      return subtrees.length > 0 || children.length > 0 ? subtrees : null
+      return (await Promise.all(children)).filter(isNotNull)
     },
     map: x => x,
     reduce: ({ context: con, loaded }, z) => {