fix @embedFile("") not giving a proper error

Currently, in a debug build of the compiler, `@embedFile("")` is a crash;
in a release build the compiler, `@embedFile("")` is "error: unable to open '': OutOfMemory".

test/cases/compile_errors/@embedFile_with_empty_path.zig

@@ -0,0 +1,11 @@
+const resource = @embedFile("");
+
+export fn entry() usize {
+    return @sizeOf(@TypeOf(resource));
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:29: error: file path name cannot be empty