Parse out empty params from queries

If an empty param,`()`, is included in a query, the boolen query parser included from advanced search gem throws an error.

It is documented as an issue upstream at projectblacklight/blacklight_advanced_search#101, but at the moment no fix upstream is proposed.

This PR works around the issue by removing empty parens from the query string. It does not impact parens containing content.

app/models/search_builder.rb

@@ -106,7 +106,7 @@ def process_is(value, op)
   end
 
   def substitute_special_chars(value, _)
-    value.gsub(/[:?]/, " ") rescue value
+    value.gsub(/([:?]|\(\))/, " ") rescue value
   end
 
   def no_journals(solr_parameters)

spec/models/search_builder_spec.rb

@@ -183,6 +183,14 @@
       # @see BL-1301 for ref.  Basically Solr treats ? as a special character.
       expect(subject.substitute_special_chars("foo bar?", nil)).to eq("foo bar ")
     end
+
+    it "substitutes empty parens '()' " do
+      expect(subject.substitute_special_chars("foo () bar", nil)).to eq("foo   bar")
+    end
+
+    it "does not substitutes parens containing values " do
+      expect(subject.substitute_special_chars("foo (bar) baz", nil)).to eq("foo (bar) baz")
+    end
   end
 
   describe "#blacklight_params" do