[TECH-DEBT] Use Empty BuildSpec in `BuildSpecFromFlags` method

From code review:

> Seems a bit a weird to me to initialize everything with pointers to empty strings just to be later updated by flag values, if I understand it correctly. Is there any way we can simplify this? What do you think?

_Originally posted by @HeavyWombat in https://github.com/shipwright-io/cli/pull/304#discussion_r2010232677_
          