fix: Skip WHERE when computed search conditions are empty

[madalinoprea]

Given an Active Record model with PK of type UUID:

+class CreateCardTransactions < ActiveRecord::Migration[6.0]
+  def change
+    create_table :card_transactions, id: :uuid do |t|
+      t.timestamps
+    end
+  end
+end

And with a Forest collection with search_fields configured only for id
field:

module Forest
  class CardTransaction
    include ForestLiana::Collection

    collection :Card__Transaction

    search_fields %w[id]
  end
end

SearchQueryBuilder will generate an invalid SQL query with an empty
WHERE statement if search term doesn't meet REGEX_UUID format:

SELECT
    "transactions"."id" AS t0_r0,
    "transactions"."processed_at" AS
FROM "transactions"
WHERE  ORDER BY "transactions"."created_at"
DESC LIMIT $1 OFFSET $2  [["LIMIT", 10], ["OFFSET", 0]]

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