descriptor: assert we never parse a sane miniscript with no pubkey

src/script/descriptor.cpp

@@ -15,6 +15,7 @@
 #include <common/args.h>
 #include <span.h>
 #include <util/bip32.h>
+#include <util/check.h>
 #include <util/spanparsing.h>
 #include <util/strencodings.h>
 #include <util/vector.h>
@@ -1566,6 +1567,9 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
                 }
                 return nullptr;
             }
+            // A signature check is required for a miniscript to be sane. Therefore no sane miniscript
+            // may have an empty list of public keys.
+            CHECK_NONFATAL(!parser.m_keys.empty());
             return std::make_unique<MiniscriptDescriptor>(std::move(parser.m_keys), std::move(node));
         }
     }