title: [electron-builder-nsis How Can I Use If-Else To Control MUI_PAGE_DIRECTORY?
body: <p>First, Forgive me for my poor English~
  Hello, I need finish this question:
  If user installed my application, update to application, I want to the install program no show the MUI_PAGE_DIRECTORY, if  people never installed, can show MUI_PAGE_DIRECTORY.
  But Found out I couldn't do it when I write install.nsh.
  such as</p>
  <pre><code>// I want this write
  ${if} $userInstallPath != &quot;&quot;
     //
  ${else}
      !insertmacro MUI_PAGE_DIRECTORY
  ${endif}
  </code></pre>
  <p>But that's not true</p>