feat: add birthday validation

src/structure/language/de-DE/de-DE.com_sportsmanager.ini

@@ -925,3 +925,5 @@ COM_SPORTSMANAGER_RANK="Rang"
 
 ; SPIELER BEARBEITEN
 COM_SPORTSMANAGER_LIZENZ="Lizenz"
+
+COM_SPORTSMANAGER_ERROR_BIRTHDAY_REQUIRED="Geburtstagsfelder dürfen nicht leer sein"

src/structure/language/en-GB/en-GB.com_sportsmanager.ini

@@ -921,3 +921,5 @@ COM_SPORTSMANAGER_NON_SMOKING_PROTECTION_MARK_NO=" (Kein Nichtraucherschutz)"
 COM_SPORTSMANAGER_POINTS_TABLE="Points table"
 COM_SPORTSMANAGER_EVALUATION="Auswertung"
 COM_SPORTSMANAGER_FUNCTION_DESCRIPTION="Variables: n = number of participants, p = place, m = multiplier of rating and in doubles possibly additionally reduced rating<br />Functions: +, -, *, /, round(x), pow(x), if(a > b, x, y), min(x, y), max(x, y), log(x), ln(x), logn(b, x)<br />VerteilungR(r, p, n, m) := max(round((((m * r - 1) * (-log(p / n) * (1 - (p / n)))) / (-log(1 / n) * (1 - (1 / n)))) + 1), 1)<br />Verteilung(r, p, n, m) := max(round(m * round((((r - 1) * (-log(p / n) * (1 - (p / n)))) / (-log(1 / n) * (1 - (1 / n)))) + 1)), 1)<br /><br />The functions VerteilungR() and Verteilung() distribute points for place 1 (r) descending to the individual places (p) of the number of participants (n).<br />VerteilungR() applies the multiplier (m) to the points for 1st place and then distributes down to 1 point for the last place.<br />Verteilung() applies the multiplier (m) to the points after the calculation, i.e. the last place receives 1 * m points."
+COM_SPORTSMANAGER_LIZENZ="License"
+COM_SPORTSMANAGER_ERROR_BIRTHDAY_REQUIRED="Birthday fields may not be empty"