hclsyntax: If nested block doesn't parse, produce empty body

Our usual rule for parse errors is to return a valid-but-incomplete object
along with error diagnostics. This was violating that rule by returning
a nil child body, which callers do not expect to deal with.

Instead, we'll return an *empty* body, so that callers who use the partial
result for careful analyses can still process the block header, without
needing to guard against the body being nil.

hcl/hclsyntax/parser.go

@@ -273,7 +273,10 @@ Token:
 			return &Block{
 				Type:   blockType,
 				Labels: labels,
-				Body:   nil,
+				Body:   &Body{
+					SrcRange: ident.Range,
+					EndRange: ident.Range,
+				},
 
 				TypeRange:       ident.Range,
 				LabelRanges:     labelRanges,