BIP62: Make OP_0 a validly encoded signature

Previously BIP62 did not provide a compact way to delibrately encode an
invalid signature. For example in BIP19 if m != n with this change you
can provide compact OP_0's in the scriptSig rather than lengthy
DER-encoded signatures.

Note that we may want to further expand on this change in the future by
saying that only OP_0 is a "valid" invalid signature; BIP19 even with
this change is inherently malleable as the invalid signatures can be any
validly encoded DER signature.

bip-0062.mediawiki

@@ -38,7 +38,7 @@ The first six and part of the seventh can be fixed by extra consensus rules, but
 ===New rules===
 
 Seven extra rules are introduced, to combat exactly the seven first sources of malleability listed above:
-# '''Canonically encoded ECDSA signatures''' An ECDSA signature passed to OP_CHECKSIG, OP_CHECKSIGVERIFY, OP_CHECKMULTISIG or OP_CHECKMULTISIGVERIFY must be encoded using strict DER encoding. Doing a verification with a non-DER signature makes the entire script evaluate to False (not just the signature verification). See reference: [[#der-encoding|DER encoding]].
+# '''Canonically encoded ECDSA signatures''' An ECDSA signature passed to OP_CHECKSIG, OP_CHECKSIGVERIFY, OP_CHECKMULTISIG or OP_CHECKMULTISIGVERIFY must be encoded using strict DER encoding. To provide a compact way to delibrately create an invalid signature for with OP_CHECKSIG and OP_CHECKMULTISIG the empty byte array (the result of OP_0) is also allowed. Doing a verification with a non-DER signature makes the entire script evaluate to False (not just the signature verification). See reference: [[#der-encoding|DER encoding]].
 # '''Non-push operations in scriptSig''' Only data pushes are allowed in scriptSig. Evaluating any other operation makes the script evaluate to false. See reference: [[#push-operators|Push operators]].
 # '''Push operations in scriptSig of non-standard size type''' The smallest possible push operation must be used when possible. Pushing data using an operation that could be encoded in a shorter way makes the script evaluate to false. See reference: [[#push-operators|Push operators]].
 # '''Zero-padded number pushes''' Any time a script opcode consumes a stack value that is interpreted as a number, it must be encoded in its shortest possible form. 'Negative zero' is not allowed. See reference: [[#numbers|Numbers]].