Fix palette fallback wrongly treating shadow palettes as missing

The is[missing] operator returns true for shadow tiddlers, so the fallback
introduced for missing palettes made every plugin palette compile as Vanilla

core/wiki/macros/CSS.tid

@@ -81,7 +81,7 @@ tags: $:/tags/Macro
 
 \procedure actions-recompile-current-palette()
 \procedure tv-action-refresh-policy() always
-<$transclude $variable="actions-compile-palette" inputPalette={{{ [{$:/palette}!is[missing]] :else[[$:/palettes/Vanilla]] }}} outputPalette="$:/temp/palette-colours"/>
+<$transclude $variable="actions-compile-palette" inputPalette={{{ [{$:/palette}is[tiddler]] [{$:/palette}is[shadow]] :else[[$:/palettes/Vanilla]] }}} outputPalette="$:/temp/palette-colours"/>
 \end actions-recompile-current-palette
 
 \procedure tv-palette-name() $:/temp/palette-colours

editions/test/tiddlers/tests/data/palettes/ShadowPaletteCompile.tid

@@ -0,0 +1,22 @@
+title: Operators/Palettes/ShadowPaletteCompile
+description: A shadow palette from a plugin compiles as itself, not the Vanilla fallback
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\import [subfilter{$:/core/config/GlobalImportFilter}]
+(<<colour background>>)
++
+title: Actions
+
+\import [subfilter{$:/core/config/GlobalImportFilter}]
+<<actions-recompile-current-palette>>
++
+title: $:/palette
+
+$:/palettes/TwentyTwenties
++
+title: ExpectedResult
+
+<p>(#FFFCF0)</p>