Add test case for handling of empty term lists in filter.

A difference between the existing test cases for the viewFilter service
and the actual inputs that are created by
`searchFilter#generateFacetedFilter` is that the actual input filter
object has empty term lists for fields that do not appear in the query.
Fix handling of this and add a test.

src/sidebar/test/view-filter-test.coffee

@@ -177,3 +177,17 @@ describe 'viewFilter', ->
         result = viewFilter.filter [annotation], filters
         assert.equal result.length, 1
         assert.equal result[0], 1
+
+  it 'ignores filters with no terms in the query', ->
+    ann = { id: 1, tags: ['foo'] }
+    filters =
+      any:
+        terms: ['foo']
+        operator: 'and'
+      tag:
+        terms: []
+        operator: 'and'
+
+    result = viewFilter.filter [ann], filters
+
+    assert.deepEqual result, [1]

src/sidebar/view-filter.js

@@ -146,7 +146,9 @@ function viewFilter(unicode) {
 
     // Convert the input filter object into a filter tree, expanding "any"
     // filters.
-    var fieldFilters = Object.entries(filters).map(([field, filter]) => {
+    var fieldFilters = Object.entries(filters).filter(([, filter]) =>
+      filter.terms.length > 0)
+    .map(([field, filter]) => {
       var terms = filter.terms.map(normalize);
       var termFilters;
       if (field === 'any') {