fix: escape quotes failing when value is not a string

[gabrielmfern]

This is meant to address resend/react-email#1632.

The issue was that the escapeQuotes was assuming that the received value
would always be a string, whereas it could be a number based on how React
treats values for styles. So even something like fontWeight: 600 is a valid
style while still not being converted into font-weight: 600px when converted.

The fix was very simple, only checking for quotes to escape if the passed in value
is actually a string.