Feeds tamper to find element in text and replace it with part of the element or 0
You can add a Tamper plugin of type "Set value or default value". Set "Value" to "0" and enable the "Only if empty" option (see image below). This way, the value "0" will be set only if the "Find replace REGEX" returned an empty result.

I tested with your example data if this would work and in my case it did.
Birth 1869 Location Merthyr Tydfil, Glamorgan, Wales&lt;a class="gs-pl" href="https://maps.google.com/?t=m&amp;amp;q=51.7479,-3.37779" target="_top" title="Google"&gt;G&lt;/a&gt; resulted into 51.7479 and Marriage Calculated 1894 resulted into 0.