FlatTermSelector: Be more defensive about termIds

It's possible (likely through a bug elsehwere) for FlatTermSelector to
completely crash the editor if it can't get termIds from the post being
edited. This change prevents that crash.

The error can happen when <FlatTermSelector> tries to access
_termIds.join and _termIds.length even when _termIds is undefined.
This is undefined if the current editor state doesn't have any
changed for the relevent attribute [0] and the original post doesn't
have the relevent attribute [1].

[0]
https://github.com/WordPress/gutenberg/blob/4da057ecd6958d40b8f2a83db1152320e5f161e6/packages/editor/src/store/selectors.js#L329
[1]
https://github.com/WordPress/gutenberg/blob/4da057ecd6958d40b8f2a83db1152320e5f161e6/packages/editor/src/store/selectors.js#L276

packages/editor/src/components/post-taxonomies/flat-term-selector.js

@@ -86,7 +86,7 @@ export function FlatTermSelector( { slug } ) {
 
 			const query = {
 				...DEFAULT_QUERY,
-				include: _termIds.join( ',' ),
+				include: _termIds?.join( ',' ),
 				per_page: -1,
 			};
 
@@ -103,7 +103,7 @@ export function FlatTermSelector( { slug } ) {
 					: false,
 				taxonomy: _taxonomy,
 				termIds: _termIds,
-				terms: _termIds.length
+				terms: _termIds?.length
 					? getEntityRecords( 'taxonomy', slug, query )
 					: EMPTY_ARRAY,
 				hasResolvedTerms: hasFinishedResolution( 'getEntityRecords', [