Make convertToSnakeCase handle edge cases

This commit updates the convertToSnakeCase utility function to handle
edge cases, in particular cases where the input only contains numbers.

app/assets/javascripts/lib/utils/text_utility.js

@@ -168,7 +168,7 @@ export const convertToCamelCase = string =>
  * @param {*} string
  */
 export const convertToSnakeCase = string =>
-  slugifyWithUnderscore(string.match(/([a-zA-Z][^A-Z]*)/g).join(' '));
+  slugifyWithUnderscore((string.match(/([a-zA-Z][^A-Z]*)/g) || [string]).join(' '));
 
 /**
  * Converts a sentence to lower case from the second word onwards

spec/frontend/lib/utils/text_utility_spec.js

@@ -126,6 +126,8 @@ describe('text_utility', () => {
       ${'snake case'}          | ${'snake_case'}
       ${'snake_case'}          | ${'snake_case'}
       ${'snakeCasesnake Case'} | ${'snake_casesnake_case'}
+      ${'123'}                 | ${'123'}
+      ${'123 456'}             | ${'123_456'}
     `('converts string $txt to $result string', ({ txt, result }) => {
       expect(textUtils.convertToSnakeCase(txt)).toEqual(result);
     });