Divide by 0 in gas computation of `(concat)`

[CryptoPascal31]

Issue description

When using (concat) with an empty list, a divide by 0 error is thrown here:

pact/src/Pact/Gas/Table.hs

Line 270 in 9d8d097

(fromIntegral nChars * charCost) `div` fromIntegral nStrings

This bug as been introduced in Pact 4.8 by: #1269

Steps to reproduce

pact> (concat [])
""

OK

---

but:

pact> (env-gasmodel "table")
pact> (env-gaslimit 10000)
"Set gas limit to 10000"
pact> (concat [])
<interactive>:1:0:Error: divide by zero

And of course, the error occurs on-chain too.

[rsoeldner]

Good catch, thank you @CryptoPascal31

[CryptoPascal31]

#1317