Allow empty import array for wasm-c-api wasm_instance_new (#782)

Allow empty import array (but not NULL) for wasm-c-api wasm_instance_new,
which makes the code more robust, especially in use cases where imports
might be constructed dynamically.
And add a test case in samples/wasm-c-api/src/empty_imports.c to test it.

samples/wasm-c-api/CMakeLists.txt

@@ -123,6 +123,7 @@ set(MM_UTIL src/utils/multi_module_utils.c)
 set(EXAMPLES
   callback
   callback_chain
+  empty_imports
   global
   hello
   hostref