* fixed nimsuggest crash with 'Something = concept' put (erroneously) outside of a 'type' block (#23331)

compiler/parser.nim

@@ -1427,7 +1427,7 @@ proc parseTypeDesc(p: var Parser, fullExpr = false): PNode =
       result = newNodeP(nkObjectTy, p)
       getTok(p)
     of tkConcept:
-      result = nil
+      result = p.emptyNode
       parMessage(p, "the 'concept' keyword is only valid in 'type' sections")
     of tkVar: result = parseTypeDescKAux(p, nkVarTy, pmTypeDesc)
     of tkOut: result = parseTypeDescKAux(p, nkOutTy, pmTypeDesc)

nimsuggest/tests/tconcept1.nim

@@ -0,0 +1,12 @@
+SomeNumber = concept
+
+#[!]#
+discard """
+$nimsuggest --tester $file
+>chk $1
+chk;;skUnknown;;;;Hint;;???;;0;;-1;;">> (toplevel): import(dirty): tests/tconcept1.nim [Processing]";;0
+chk;;skUnknown;;;;Error;;$file;;1;;13;;"the \'concept\' keyword is only valid in \'type\' sections";;0
+chk;;skUnknown;;;;Error;;$file;;1;;13;;"invalid indentation";;0
+chk;;skUnknown;;;;Error;;$file;;1;;13;;"expression expected, but found \'keyword concept\'";;0
+chk;;skUnknown;;;;Error;;$file;;1;;0;;"\'SomeNumber\' cannot be assigned to";;0
+"""

nimsuggest/tests/tconcept2.nim

@@ -0,0 +1,15 @@
+  SomeNumber = concept a, type T
+    a.int is int
+    int.to(T) is type(a)
+
+#[!]#
+discard """
+$nimsuggest --tester $file
+>chk $1
+chk;;skUnknown;;;;Hint;;???;;0;;-1;;">> (toplevel): import(dirty): tests/tconcept2.nim [Processing]";;0
+chk;;skUnknown;;;;Error;;$file;;1;;2;;"invalid indentation";;0
+chk;;skUnknown;;;;Error;;$file;;1;;15;;"the \'concept\' keyword is only valid in \'type\' sections";;0
+chk;;skUnknown;;;;Error;;$file;;1;;15;;"invalid indentation";;0
+chk;;skUnknown;;;;Error;;$file;;1;;15;;"expression expected, but found \'keyword concept\'";;0
+chk;;skUnknown;;;;Error;;$file;;1;;2;;"\'SomeNumber\' cannot be assigned to";;0
+"""