fix(language-core): do not emit undefined for model with falsy default value

[RylanBueckert-Broadsign]

vuejs/core#12817 (comment)

[pkg-pr-new]

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vue-component-meta

npm i https://pkg.pr.new/vuejs/language-tools/vue-component-meta@5198

vue-component-type-helpers

npm i https://pkg.pr.new/vuejs/language-tools/vue-component-type-helpers@5198

@vue/language-core

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-core@5198

@vue/language-plugin-pug

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-plugin-pug@5198

@vue/language-server

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-server@5198

@vue/language-service

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-service@5198

vue-tsc

npm i https://pkg.pr.new/vuejs/language-tools/vue-tsc@5198

@vue/typescript-plugin

npm i https://pkg.pr.new/vuejs/language-tools/@vue/typescript-plugin@5198

commit: 4ceea27