How to make a column span full width when a second column is not there? (CSS Grid)

Don't define the columns explicitly with grid-template-columns.

Make the columns implicit instead and then use grid-auto-columns to define their widths.

This will allow the first column (.content) to consume all space in the row when the second column (.sidebar) doesn't exist.

.grid {
  display: grid;
  grid-auto-columns: 1fr 200px;
}

.content {
  grid-column: 1;
}

.sidebar {
  grid-column: 2;
}

.grid > * {
  border: 1px dashed red; /* demo only */
}

<p>With side bar:</p>

<div class="grid">

  <div class="content">
    <p>content</p>
  </div>
  
  <div class="sidebar">
    <p>sidebar</p>
  </div>

</div>

<p>No side bar:</p>

<div class="grid">

  <div class="content">
    <p>content</p>
  </div>
  
</div>

You can get closer by using content sizing keywords, something like:

.grid {
  display: grid;
  grid-template-columns: 1fr fit-content(200px);
}

.sidebar {
  width: 100%;
}

The fit-content keyword will look at the size of the content and act like max-content until it gets to the value you pass in.

In reality you probably wouldn't need to stick a size on sidebar as the content is likely to dictate a size of at least 200 pixels (for example) but you can play around with this.

I think I know the definitive answer to this question now. The problem with the answers so far is that they don't explain how to handle a sidebar that is on the left side of the main content (mainly because I didn't ask for it in the original question).

<div class="grid">

  <nav>
    <p>navigation</p>
  </nav>

  <main>
    <p>content</p>
  </main>

  <aside>
    <p>sidebar</p>
  </aside>

</div>

You can use this CSS:

.grid {
  display: grid;
  grid-template-columns: fit-content(200px) 1fr fit-content(200px);
}

nav, aside {
  width: 100%;
}

/* ensures that the content will always be placed in the correct column */
nav { grid-column: 1; }
main { grid-column: 2; }
aside { grid-column: 3; }

This is also a good use case for grid-areas

.grid {
  display: grid;
  grid-template-columns: fit-content(200px) 1fr fit-content(200px);
  grid-template-areas: "nav content sidebar";
}

nav, aside {
  width: 100%;
}

/* ensures that the content will always be placed in the correct column */
nav { grid-area: nav; }
main { grid-area: content; }
aside { grid-area: sidebar; }

An IE compatible version would look like this:

.grid {
  display: -ms-grid;
  display: grid;
  -ms-grid-columns: auto 1fr auto;
  grid-template-columns: auto 1fr auto;
}

nav, aside {
  width: 100%; /* Ensures that if the content exists, it takes up max-width */
  max-width: 200px; /* Prevents the content exceeding 200px in width */
}

/* ensures that the content will always be placed in the correct column */
nav {
  -ms-grid-column: 1;
  grid-column: 1;
}

main {
  -ms-grid-column: 2;
  grid-column: 2;
}

aside {
  -ms-grid-column: 3;
  grid-column: 3;
}