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/src/empty_imports.wat

@@ -0,0 +1,5 @@
+(module
+  (func (export "add") (param i32 i32) (result i32)
+    (i32.add (local.get 0) (local.get 1))
+  )
+)