Fix single column block display for smaller screens. (#26438)

If there is only one column, don't force a 50% flex-basis for small screens.

packages/block-library/src/columns/style.scss

@@ -47,11 +47,14 @@
 
 	// Between mobile and large viewports, allow 2 columns.
 	@media (min-width: #{ ($break-small) }) and (max-width: #{ ($break-medium - 1) }) {
-		// As with mobile styles, this must be important since the Column
-		// assigns its own width as an inline style, which should take effect
-		// starting at `break-medium`.
-		flex-basis: calc(50% - 1em) !important;
-		flex-grow: 0;
+		// Only add two column styling if there are two or more columns
+		&:not(:only-child) {
+			// As with mobile styles, this must be important since the Column
+			// assigns its own width as an inline style, which should take effect
+			// starting at `break-medium`.
+			flex-basis: calc(50% - 1em) !important;
+			flex-grow: 0;
+		}
 
 		// Add space between the multiple columns. Themes can customize this if they wish to work differently.
 		// Only apply this beyond the mobile breakpoint, as there's only a single column on mobile.